vA mathematician knows how to solve a problem,
he can not solve it. – MILNE v
3.1 Introduction
The word ‘trigonometry’ is derived from the Greek words
‘trigon’ and ‘metron’ and it means ‘measuring the sides of
a triangle’. The subject was originally developed to solve
geometric problems involving triangles. It was studied by
sea captains for navigation, surveyor to map out the new
lands, by engineers and others. Currently, trigonometry is
used in many areas such as the science of seismology,
designing electric circuits, describing the state of an atom,
predicting the heights of tides in the ocean, analysing a
musical tone and in many other areas.
In earlier classes, we have studied the trigonometric
ratios of acute angles as the ratio of the sides of a right
angled triangle. We have also studied the trigonometric identities and application of
trigonometric ratios in solving the problems related to heights and distances. In this
Chapter, we will generalise the concept of trigonometric ratios to trigonometric functions
and study their properties.
3.2 Angles
Angle is a measure of rotation of a given ray about its initial point. The original ray is
Chapter
3
TRIGONOMETRIC FUNCTIONS
Arya Bhatt
(476-550)
Fig 3.1
Vertex
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50 MATHEMATICS
called the initial side and the final position of the ray after rotation is called the
terminal side of the angle. The point of rotation is called the vertex. If the direction of
rotation is anticlockwise, the angle is said to be positive and if the direction of rotation
is clockwise, then the angle is negative (Fig 3.1).
The measure of an angle is the amount of
rotation performed to get the terminal side from
the initial side. There are several units for
measuring angles. The definition of an angle
suggests a unit, viz. one complete revolution from the position of the initial side as
indicated in Fig 3.2.
This is often convenient for large angles. For example, we can say that a rapidly
spinning wheel is making an angle of say 15 revolution per second. We shall describe
two other units of measurement of an angle which are most commonly used, viz.
degree measure and radian measure.
3.2.1
Degree measure
If a rotation from the initial side to terminal side is
th
1
360
of
a revolution, the angle is said to have a measure of one degree, written as 1°. A degree is
divided into 60 minutes, and a minute is divided into 60 seconds . One sixtieth of a degree is
called a minute, written as 1′, and one sixtieth of a minute is called a second, written as 1″.
Thus, 1° = 60′, 1′ = 60″
Some of the angles whose measures are 360°,180°, 270°, 420°, – 30°, – 420° are
shown in Fig 3.3.
Fig 3.2
Fig 3.3
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TRIGONOMETRIC FUNCTIONS 51
3.2.2 Radian measure
There is another unit for measurement of an angle, called
the radian measure. Angle subtended at the centre by an arc of length 1 unit in a
unit circle (circle of radius 1 unit) is said to have a measure of 1 radian. In the Fig
3.4(i) to (iv), OA is the initial side and OB is the terminal side. The figures show the
angles whose measures are 1 radian, –1 radian, 1
1
2
radian and –1
1
2
radian.
(i)
(ii)
(iii)
Fig 3.4 (i) to (iv)
(iv)
We know that the circumference of a circle of radius 1 unit is 2π. Thus, one
complete revolution of the initial side subtends an angle of 2π radian.
More generally, in a circle of radius r, an arc of length r will subtend an angle of
1 radian. It is well-known that equal arcs of a circle subtend equal angle at the centre.
Since in a circle of radius r, an arc of length r subtends an angle whose measure is 1
radian, an arc of length l will subtend an angle whose measure is
l
r
radian. Thus, if in
a circle of radius r, an arc of length l subtends an angle θ radian at the centre, we have
θ =
l
r
or l = r θ.
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52 MATHEMATICS
3.2.3 Relation between radian and real numbers
Consider the unit circle with centre O. Let A be any point
on the circle. Consider OA as initial side of an angle.
Then the length of an arc of the circle will give the radian
measure of the angle which the arc will subtend at the
centre of the circle. Consider the line PAQ which is
tangent to the circle at A. Let the point A represent the
real number zero, AP represents positive real number and
AQ represents negative real numbers (Fig 3.5). If we
rope the line AP in the anticlockwise direction along the
circle, and AQ in the clockwise direction, then every real
number will correspond to a radian measure and
conversely. Thus, radian measures and real numbers can
be considered as one and the same.
3.2.4
Relation between degree and radian
Since a circle subtends at the centre
an angle whose radian measure is 2π and its degree measure is 360°, it follows that
2π radian = 360° or π radian = 180°
The above relation enables us to express a radian measure in terms of degree
measure and a degree measure in terms of radian measure. Using approximate value
of π as
22
7
, we have
1 radian =
180
π
°
= 57° 16′ approximately.
Also 1° =
π
180
radian = 0.01746 radian approximately.
The relation between degree measures and radian measure of some common angles
are given in the following table:
A
O
1
P
1
2
−1
−2
Q
0
Fig 3.5
Degree 30° 45° 60° 90° 180° 270°
360°
Radian
π
6
π
4
π
3
π
2
π
3
π
2
2
π
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TRIGONOMETRIC FUNCTIONS 53
Notational Convention
Since angles are measured either in degrees or in radians, we adopt the convention
that whenever we write angle θ°, we mean the angle whose degree measure is θ and
whenever we write angle β, we mean the angle whose radian measure is β.
Note that when an angle is expressed in radians, the word ‘radian’ is frequently
omitted. Thus,
π
π 180 and 45
4
= ° = °
are written with the understanding that π
and
π
4
are radian measures. Thus, we can say that
Radian measure =
π
180
×
Degree measure
Degree measure =
180
π
×
Radian measure
Example 1 Convert 40° 20′ into radian measure.
Solution We know that 180° = π radian.
Hence 40° 20′ = 40
1
3
degree =
π
180
×
121
3
radian =
121
π
540
radian.
Therefore 40° 20′ =
121
π
540
radian.
Example 2 Convert 6 radians into degree measure.
Solution We know that π radian = 180°.
Hence 6 radians =
180
π
×
6 degree =
1080
7
22
×
degree
= 343
7
11
degree = 343° +
7
60
11
×
minute [as 1° = 60′]
= 343° + 38′ +
2
11
minute [as 1′ = 60″]
= 343° + 38′ + 10.9″ = 343°38′ 11″ approximately.
Hence 6 radians = 343° 38′ 11″ approximately.
Example 3 Find the radius of the circle in which a central angle of 60° intercepts an
arc of length 37.4 cm (use
22
π
7
=
).
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54 MATHEMATICS
Solution Here l
= 37.4 cm and θ = 60° =
60
π π
radian =
180 3
Hence, by r =
θ
l
, we have
r =
37.4×3 37.4×3×7
=
π 22
= 35.7 cm
Example 4 The minute hand of a watch is 1.5 cm long. How far does its tip move in
40 minutes? (Use π = 3.14).
Solution In 60 minutes, the minute hand of a watch completes one revolution. Therefore,
in 40 minutes, the minute hand turns through
2
3
of a revolution. Therefore,
2
θ =
× 360°
3
or
4
π
3
radian. Hence, the required distance travelled is given by
l = r θ = 1.5
×
4
π
3
cm = 2π cm = 2
×
3.14 cm = 6.28 cm.
Example 5 If the arcs of the same lengths in two circles subtend angles 65°and 110°
at the centre, find the ratio of their radii.
Solution Let r
1
and r
2
be the radii of the two circles. Given that
θ
1
= 65° =
π
65
180
×
=
13
π
36
radian
and θ
2
= 110° =
π
110
180
×
=
22
π
36
radian
Let l be the length of each of the arc. Then l
= r
1
θ
1
= r
2
θ
2
, which gives
13
π
36
×
r
1
=
22
π
36
×
r
2
,
i.e.,
1
2
r
r
=
22
13
Hence r
1
: r
2
= 22 : 13.
EXERCISE 3.1
1. Find the radian measures corresponding to the following degree measures:
(i) 25° (ii) – 47°30′ (iii) 240° (iv) 520°
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TRIGONOMETRIC FUNCTIONS 55
2. Find the degree measures corresponding to the following radian measures
(Use
22
π
7
=
).
(i)
11
16
(ii) – 4 (iii)
5
π
3
(iv)
7
π
6
3. A wheel makes 360 revolutions in one minute. Through how many radians does
it turn in one second?
4. Find the degree measure of the angle subtended at the centre of a circle of
radius 100 cm by an arc of length 22 cm (Use
22
π
7
=
).
5. In a circle of diameter 40 cm, the length of a chord is 20 cm. Find the length of
minor arc of the chord.
6. If in two circles, arcs of the same length subtend angles 60° and 75° at the
centre, find the ratio of their radii.
7. Find the angle in radian through which a pendulum swings if its length is 75 cm
and th e tip describes an arc of length
(i) 10 cm (ii) 15 cm (iii) 21 cm
3.3 Trigonometric Functions
In earlier classes, we have studied trigonometric ratios for acute angles as the ratio of
sides of a right angled triangle. We will now extend the definition of trigonometric
ratios to any angle in terms of radian measure and study them as trigonometric functions.
Consider a unit circle with centre
at origin of the coordinate axes. Let
P (a, b) be any point on the circle with
angle AOP = x radian, i.e., length of arc
AP =
x (Fig 3.6).
We define cos x
= a and sin x = b
Since ∆OMP is a right triangle, we have
OM
2
+ MP
2
= OP
2
or a
2
+ b
2
= 1
Thus, for every point on the unit circle,
we have
a
2
+ b
2
= 1 or cos
2
x + sin
2
x = 1
Since one complete revolution
subtends an angle of 2π radian at the
centre of the circle, ∠AOB =
π
2
,
Fig 3.6
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56 MATHEMATICS
∠AOC = π and ∠AOD =
3
π
2
. All angles which are integral multiples of
π
2
are called
quadrantal angles. The coordinates of the points A, B, C and D are, respectively,
(1, 0), (0, 1), (–1, 0) and (0, –1). Therefore, for quadrantal angles, we have
cos 0° = 1 sin 0° = 0,
cos
π
2
= 0 sin
π
2
= 1
cosπ = − 1 sinπ = 0
cos
3
π
2
= 0 sin
3
π
2
= –1
cos 2π = 1 sin 2π = 0
Now, if we take one complete revolution from the point P, we again come back to
same point P. Thus, we also observe that if x increases (or decreases) by any integral
multiple of 2π, the values of sine and cosine functions do not change. Thus,
sin (2nπ + x) = sin
x
,
n ∈
Z , cos (2nπ + x) = cos
x
,
n ∈
Z
Further, sin x = 0, if
x
= 0, ± π, ± 2π , ± 3π, ..., i.e., when x is an integral multiple of π
and cos x = 0, if
x
= ±
π
2
, ±
3
π
2
, ±
5
π
2
, ... i.e., cos x vanishes when x is an odd
multiple of
π
2
. Thus
sin x = 0 implies x = n
π, π,
π, π,
π, where n is any integer
cos x = 0 implies x = (2n + 1)
π
2
, where n is any integer.
We now define other trigonometric functions in terms of sine and cosine functions:
cosec x =
1
sin
x
, x ≠
nπ, where n is any integer.
sec x =
1
cos
x
, x ≠ (2n + 1)
π
2
, where n is any integer.
tan x =
sin
cos
x
x
, x ≠ (2n +1)
π
2
, where n is any integer.
cot x =
cos
sin
x
x
, x ≠ n π, where n is any integer.
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TRIGONOMETRIC FUNCTIONS 57
not
defined
not
defined
We have shown that for all real x, sin
2
x + cos
2
x = 1
It follows that
1 + tan
2
x = sec
2
x (why?)
1 + cot
2
x = cosec
2
x (why?)
In earlier classes, we have discussed the values of trigonometric ratios for 0°,
30°, 45°, 60° and 90°. The values of trigonometric functions for these angles are same
as that of trigonometric ratios studied in earlier classes. Thus, we have the following
table:
0°
π
6
π
4
π
3
π
2
π
3
π
2
2
π
sin 0
1
2
1
2
3
2
1 0 – 1 0
cos 1
3
2
1
2
1
2
0 – 1 0 1
tan 0
1
3
1
3
0 0
The values of cosec x, sec x and cot x
are the reciprocal of the values of sin x,
cos x and tan x, respectively.
3.3.1 Sign of trigonometric functions
Let P (a, b) be a point on the unit circle
with centre at the origin such that
∠AOP = x
.
If ∠AOQ = – x, then the
coordinates of the point Q will be (a, –b)
(Fig 3.7). Therefore
cos (– x) = cos x
and sin (– x) = – sin x
Since for every point P (a, b) on
the unit circle, – 1 ≤ a ≤ 1 and
Fig 3.7
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58 MATHEMATICS
– 1 ≤ b ≤ 1, we have – 1 ≤ cos x ≤ 1 and –1
≤ sin
x
≤ 1 for all
x. We have learnt in
previous classes that in the first quadrant (0 < x
<
π
2
) a and
b are both positive, in the
second quadrant (
π
2
< x <π) a is negative and b is positive, in the third quadrant
(π < x <
3
π
2
) a and b are both negative and in the fourth quadrant (
3
π
2
< x < 2π) a is
positive and b is negative. Therefore, sin x is positive for 0 < x < π, and negative for
π < x < 2π. Similarly, cos x is positive for 0 < x
<
π
2
, negative for
π
2
< x
<
3
π
2
and also
positive for
3
π
2
< x < 2π. Likewise, we can find the signs of other trigonometric
functions in different quadrants. In fact, we have the following table.
I II III IV
sin x + + – –
cos x + – – +
tan x + – + –
cosec x + + – –
sec x + – – +
cot x + – + –
3.3.2 Domain and range of trigonometric functions From the definition of sine
and cosine functions, we observe that they are defined for all real numbers. Further,
we observe that for each real number x,
– 1 ≤ sin x ≤ 1 and – 1 ≤ cos x ≤ 1
Thus, domain of y = sin x and y = cos x is the set of all real numbers and range
is the interval [–1, 1], i.e., – 1 ≤ y ≤ 1.
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TRIGONOMETRIC FUNCTIONS 59
Since cosec x =
1
sin
x
, the domain of y = cosec x is the set { x :
x ∈ R and
x
≠
n π, n ∈ Z} and range is the set {y : y ∈ R, y ≥ 1 or y ≤ – 1}. Similarly, the domain
of y = sec x is the set {x : x ∈ R and x ≠ (2n + 1)
π
2
, n ∈ Z} and range is the set
{y : y ∈ R, y ≤ – 1or y ≥ 1}. The domain of y = tan x is the set {x : x ∈ R and
x ≠ (2n + 1)
π
2
, n ∈ Z} and range is the set of all real numbers. The domain of
y = cot x is the set {x : x ∈ R and x ≠ n π, n ∈ Z} and the range is the set of all real
numbers.
We further observe that in the first quadrant, as x increases from 0 to
π
2
, sin x
increases from 0 to 1, as x increases from
π
2
to π, sin x decreases from 1 to 0. In the
third quadrant, as x increases from π to
3
π
2
, sin x decreases from 0 to –1and finally, in
the fourth quadrant, sin x increases from –1 to 0 as x increases from
3
π
2
to 2π.
Similarly, we can discuss the behaviour of other trigonometric functions. In fact, we
have the following table:
Remark In the above table, the statement tan x increases from 0 to ∞ (infinity) for
0 < x <
π
2
simply means that tan x increases as x increases for 0 < x
<
π
2
and
I quadrant II quadrant III quadrant IV quadrant
sin increases from 0 to 1 decreases from 1 to 0 decreases from 0 to –1 increases from –1 to 0
cos decreases from 1 to 0 decreases from 0 to – 1 increases from –1 to 0 increases from 0 to 1
tan increases from 0 to ∞ increases from –∞to 0 increases from 0 to ∞ increases from –∞to 0
cot decreases from ∞ to 0 decreases from 0 to–∞ decreases from ∞ to
0 decreases from 0to –∞
sec increases from 1 to ∞ increases from –∞to–1 decreases from –1to–∞ decreases from ∞ to 1
cosec decreases from ∞ to 1 increases from 1 to ∞ increases from –∞to–1 decreases from–1to–∞
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60 MATHEMATICS
Fig 3.10
Fig 3.11
Fig 3.8
Fig 3.9
assumes arbitraily large positive values as x approaches to
π
2
. Similarly, to say that
cosec x decreases from –1 to – ∞ (minus infinity) in the fourth quadrant means that
cosec x decreases for x ∈
(
3
π
2
, 2π) and assumes arbitrarily large negative values as
x approaches to 2π. The symbols ∞ and – ∞ simply specify certain types of behaviour
of functions and variables.
We have already seen that values of sin x and cos x repeats after an interval of
2π. Hence, values of cosec x and sec x will also repeat after an interval of 2π. We
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TRIGONOMETRIC FUNCTIONS 61
shall see in the next section that tan (π + x) = tan x. Hence, values of tan x will repeat
after an interval of π. Since cot x is reciprocal of tan x, its values will also repeat after
an interval of π. Using this knowledge and behaviour of trigonometic functions, we can
sketch the graph of these functions. The graph of these functions are given above:
Example 6 If cos x = –
3
5
, x lies in the third quadrant, find the values of other five
trigonometric functions.
Solution Since cos x =
3
5
−
, we have sec x =
5
3
−
Now sin
2
x + cos
2
x = 1, i.e., sin
2
x = 1 – cos
2
x
or sin
2
x = 1 –
9
25
=
16
25
Hence sin x = ±
4
5
Since x lies in third quadrant, sin x is negative. Therefore
sin x = –
4
5
which also gives
cosec x = –
5
4
Fig 3.12
Fig 3.13
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62 MATHEMATICS
Further, we have
tan x =
sin
cos
x
x
=
4
3
and cot x =
cos
sin
x
x
=
3
4
.
Example 7 If cot
x = –
5
12
, x lies in second quadrant, find the values of other five
trigonometric functions.
Solution Since cot x = –
5
12
, we have tan x = –
12
5
Now sec
2
x = 1 + tan
2
x = 1 +
144
25
=
169
25
Hence sec x = ±
13
5
Since x lies in second quadrant, sec x will be negative. Therefore
sec x = –
13
5
,
which also gives
5
cos
13
x
= −
Further, we have
sin x = tan x cos x = (–
12
5
)
×
(–
5
13
) =
12
13
and cosec x =
1
sin
x
=
13
12
.
Example 8 Find the value of sin
31
π
3
.
Solution We know that values of sin x repeats after an interval of 2π. Therefore
sin
31
π
3
= sin (10π +
π
3
) = sin
π
3
=
3
2
.
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TRIGONOMETRIC FUNCTIONS 63
Example 9 Find the value of cos (–1710°).
Solution We know that values of cos x repeats after an interval of 2π or 360°.
Therefore, cos (–1710°) = cos (–1710° + 5
×
360°)
= cos (–1710° + 1800°) = cos 90° = 0.
EXERCISE 3.2
Find the values of other five trigonometric functions in Exercises 1 to 5.
1. cos x = –
1
2
, x lies in third quadrant.
2. sin x
=
3
5
, x lies in second quadrant.
3. cot x
=
4
3
, x lies in third quadrant.
4. sec x
=
13
5
, x lies in fourth quadrant.
5. tan x
= –
5
12
, x lies in second quadrant.
Find the values of the trigonometric functions in Exercises 6 to 10.
6. sin 765° 7. cosec (– 1410°)
8. tan
19
π
3
9. sin (–
11
π
3
)
10. cot (–
15
π
4
)
3.4 Trigonometric Functions of Sum and Difference of Two Angles
In this Section, we shall derive expressions for trigonometric functions of the sum and
difference of two numbers (angles) and related expressions. The basic results in this
connection are called trigonometric identities. We have seen that
1. sin (– x) = – sin x
2. cos (– x) = cos x
We shall now prove some more results:
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64 MATHEMATICS
3. cos (x
+ y) = cos x cos y – sin x sin y
Consider the unit circle with centre at the origin. Let
x
be the angle P
4
OP
1
and y be
the angle P
1
OP
2
. Then (x + y) is the angle P
4
OP
2
. Also let (– y) be the angle P
4
OP
3
.
Therefore, P
1
, P
2
, P
3
and P
4
will have the coordinates P
1
(cos x, sin
x),
P
2
[cos (x + y), sin (x + y)], P
3
[cos (– y), sin (– y)] and P
4
(1, 0) (Fig 3.14).
Consider the triangles P
1
OP
3
and P
2
OP
4
. They are congruent (Why?). Therefore,
P
1
P
3
and P
2
P
4
are equal. By using distance formula, we get
P
1
P
3
2
= [cos x – cos (– y)]
2
+ [sin x – sin(–y]
2
= (cos x – cos y)
2
+ (sin x + sin y)
2
= cos
2
x + cos
2
y – 2 cos x cos y + sin
2
x + sin
2
y + 2sin x sin
y
= 2 – 2 (cos x cos y – sin x sin y) (Why?)
Also, P
2
P
4
2
= [1 – cos (x + y)]
2
+ [0 – sin (x + y)]
2
= 1 – 2cos (x + y) + cos
2
(x + y) + sin
2
(x + y)
= 2 – 2 cos (x + y)
Fig 3.14
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TRIGONOMETRIC FUNCTIONS 65
Since P
1
P
3
= P
2
P
4
, we have P
1
P
3
2
= P
2
P
4
2
.
Therefore, 2 –2 (cos x cos y – sin x sin y) = 2 – 2 cos (x + y).
Hence cos (x + y) = cos x cos y – sin x sin y
4. cos (x – y) = cos x
cos y + sin x sin y
Replacing y by – y in identity 3, we get
cos
(
x
+ (– y)) = cos x cos (– y) – sin x sin (– y)
or cos (x
– y) = cos x cos y + sin x sin y
5. cos (
x
π
–
2
) = sin x
If we replace x by
π
2
and y by x in Identity (4), we get
cos (
π
2
x
−
) = cos
π
2
cos x
+ sin
π
2
sin x = sin x.
6. sin (
x
π
–
2
) = cos x
Using the Identity 5, we have
sin (
π
2
x
−
) = cos
π π
2 2
x
− −
= cos x.
7. sin (x
+ y) = sin x cos y + cos x sin y
We know that
sin (x
+ y) = cos
π
( )
2
x y
− +
= cos
π
( )
2
x y
− −
= cos (
π
2
x
−
) cos y + sin
π
( )
2
x
−
sin y
= sin x cos y + cos x sin y
8. sin (x
– y) = sin x cos y – cos x sin y
If we replace y by –y, in the Identity 7, we get the result.
9. By taking suitable values of x and y in the identities 3, 4, 7 and 8, we get the
following results:
cos
x
π
( + )
2
= – sin x sin
x
π
( + )
2
= cos x
cos (
ππ
ππ
π – x) = – cos x sin (
ππ
ππ
π – x) = sin x
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66 MATHEMATICS
cos (
π
π
π
π
π + x) = – cos x sin (
π
π
π
π
π + x) = – sin
x
cos (2
ππ
ππ
π – x) = cos x sin (2
ππ
ππ
π – x) = – sin x
Similar results for tan x, cot x, sec x and cosec x can be obtained from the results of sin
x and cos x.
10. If none of the angles x, y and (x
+ y) is an odd multiple of
π
2
, then
tan (x
+ y) =
x y
x y
tan + tan
1 – tan tan
Since none of the x, y and (x + y) is an odd multiple of
π
2
, it follows that cos x,
cos y and cos (x + y) are non-zero. Now
tan (x + y) =
sin(
)
cos( )
x
y
x y
+
+
=
sin
cos
cos
sin
cos cos sin sin
x
y
x
y
x y x y
+
−
.
Dividing numerator and denominator by cos x cos y, we have
tan (x
+ y) =
yx
yx
yx
yx
yx
yx
yx
yx
coscos
sinsin
coscos
coscos
coscos
sincos
coscos
cossin
−
+
=
tan tan
1 – tan tan
x y
x y
+
11. tan ( x – y) =
x y
x y
tan – tan
1 + tan tan
If we replace y by – y in Identity 10, we get
tan (x – y) = tan [x + (– y)]
=
tan tan ( )
1 tan tan ( )
x y
x y
+ −
− −
=
tan tan
1 tan tan
x y
x y
−
+
12. If none of the angles x, y and (x + y) is a multiple of
ππ
ππ
π, then
cot ( x + y) =
x y
y x
cot cot – 1
cot + cot
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TRIGONOMETRIC FUNCTIONS 67
Since, none of the x, y and (x + y) is multiple of π, we find that sin x sin y and
sin (
x + y) are non-zero. Now,
cot ( x + y) =
cos ( ) cos cos – sin sin
sin ( ) sin cos cos sin
x y x y x y
x y x y x y
+
=
+ +
Dividing numerator and denominator by sin x sin y, we have
cot (x + y) =
cot cot – 1
cot cot
x y
y x
+
13. cot (x – y) =
x y
y x
cot cot +1
cot – cot
if none of angles x, y and x–y is a multiple of π
If we replace y by –y in identity 12, we get the result
14. cos 2x = cos
2
x – sin
2
x = 2 cos
2
x – 1 = 1 – 2 sin
2
x =
x
x
2
2
1 – tan
1 + tan
We know that
cos (x + y) = cos x cos y – sin x sin y
Replacing y by x, we get
cos 2x = cos
2
x – sin
2
x
= cos
2
x – (1 – cos
2
x) = 2 cos
2
x – 1
Again, cos 2x = cos
2
x – sin
2
x
= 1 – sin
2
x – sin
2
x = 1 – 2 sin
2
x.
We have cos 2x = cos
2
x – sin
2
x =
2 2
2 2
cos sin
cos sin
x x
x x
−
+
Dividing numerator and denominator by cos
2
x, we get
cos 2x =
2
2
1 – tan
1 + tan
x
x
,
π
π
2
≠ +
x n
, where n is an integer
15. sin 2x = 2 sinx cos x =
x
x
2
2tan
1 + tan
π
π
2
≠ +
x n
, where n is an integer
We have
sin (x + y) = sin x cos y + cos x sin y
Replacing y by x, we get sin 2x = 2 sin x cos x.
Again sin 2x =
2 2
2sin cos
cos sin
x x
x x
+
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68 MATHEMATICS
Dividing each term by cos
2
x, we get
sin 2x =
2
2tan
1 tan
x
x
+
16. tan 2x =
x
x
2
2tan
1 – tan
if
π
2 π
2
≠ +
x n
, where n is an integer
We know that
tan (x + y) =
tan tan
1 tan tan
x y
– x y
+
Replacing y by x , we get
2
2 tan
tan 2
1 tan
x
x
x
=
−
17. sin 3x = 3 sin x – 4 sin
3
x
We have,
sin 3x = sin (2x + x)
= sin 2x cos x + cos 2x sin x
= 2 sin x cos x cos x + (1 – 2sin
2
x) sin x
= 2 sin x (1 – sin
2
x) + sin x – 2 sin
3
x
= 2 sin x – 2 sin
3
x + sin x – 2 sin
3
x
= 3 sin x – 4 sin
3
x
18. cos 3x= 4 cos
3
x – 3 cos x
We have,
cos 3x = cos (2x +x)
= cos 2x cos
x – sin 2x sin x
= (2cos
2
x – 1) cos x – 2sin x cos x sin x
= (2cos
2
x – 1) cos x – 2cos x (1 – cos
2
x)
= 2cos
3
x – cos x – 2cos x + 2 cos
3
x
= 4cos
3
x – 3cos x.
19.
=
x x
x
x
3
2
3 tan – tan
tan 3
1 – 3tan
if
π
3 π
2
≠ +
x n
, where n is an integer
We have tan 3x =tan (2x + x)
=
tan 2 tan
1 tan 2 tan
x x
– x x
+
2
2
2tan
tan
1 tan
2tan tan
1
1 tan
x
x
– x
x . x
–
– x
+
=
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TRIGONOMETRIC FUNCTIONS 69
3 3
2 2 2
2tan tan tan 3 tan tan
1 tan 2tan 1 3tan
x x – x x – x
– x – x – x
+
= =
20. (i) cos x + cos y =
x y x y
+ –
2cos cos
2 2
(ii) cos x – cos y = –
x y x y
+ –
2sin sin
2 2
(iii) sin x + sin y =
x y x y
+ –
2sin cos
2 2
(iv) sin x – sin y =
x y x y
+ –
2cos sin
2 2
We know that
cos (x + y) = cos x cos y – sin x sin y ... (1)
and cos (x – y) = cos x cos y + sin x sin y ... (2)
Adding and subtracting (1) and (2), we get
cos (x + y) + cos(x – y) = 2 cos x cos y ... (3)
and cos (x + y) – cos (x – y) = – 2 sin x sin y ... (4)
Further sin (
x + y) = sin x cos y + cos x sin y ... (5)
and sin (x – y) = sin x cos y – cos x sin y ... (6)
Adding and subtracting (5) and (6), we get
sin (x + y) + sin (x – y) = 2 sin x cos y ... (7)
sin (x + y) – sin (x – y) = 2cos x sin y ... (8)
Let x + y = θ and x – y = φ. Therefore
θ θ
and
2 2
x y
+φ −φ
= =
Substituting the values of x and y in (3), (4), (7) and (8), we get
cos θ + cos φ = 2 cos
θ θ
cos
2 2
+φ −φ
cos θ – cos φ = – 2 sin
θ θ
sin
2
–
+ φ φ
2
sin θ + sin φ = 2 sin
θ θ
cos
2 2
+φ −φ
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sin θ – sin φ = 2 cos
θ θ
sin
2 2
+φ −φ
Since θ and φ can take any real values, we can replace θ by x and φ by y.
Thus, we get
cos x + cos y = 2 cos
cos
2 2
x y x y
+ −
; cos x – cos y = – 2 sin
sin
2 2
x y x y
+ −
,
sin x + sin y = 2 sin
cos
2 2
x y x y
+ −
; sin x – sin y = 2 cos
sin
2 2
x y x y
+ −
.
Remark As a part of identities given in 20, we can prove the following results:
21. (i) 2 cos x cos y = cos (x + y) + cos (x – y)
(ii) –2 sin x sin y = cos (x + y) – cos (x – y)
(iii) 2 sin x cos y = sin (x + y) + sin (x – y)
(iv) 2 cos x sin y = sin (x + y) – sin (x – y).
Example 10 Prove that
5
3sin sec 4sin cot 1
6 3 6 4
π π π π
− =
Solution We have
L.H.S. =
5
3sin sec 4sin cot
6 3 6 4
π π π π
−
= 3 ×
1
2
× 2 – 4 sin
6
π
π−
× 1 = 3 – 4 sin
6
π
= 3 – 4 ×
1
2
= 1 = R.H.S.
Example 11 Find the value of sin 15°.
Solution We have
sin 15° = sin (45° – 30°)
= sin 45° cos 30° – cos 45° sin 30°
=
1 3 1 1 3 1
2 2
2 2 2 2
–
× − × =
.
Example 12 Find the value of tan
13
12
π
.
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TRIGONOMETRIC FUNCTIONS 71
Solution We have
tan
13
12
π
= tan
12
π
π +
= tan
tan
12 4 6
π π π
= −
=
tan tan
4 6
1 tan tan
4 6
π π
−
π π
+
=
1
1
3 1
3
2 3
1
3 1
1
3
−
−
= = −
+
+
Example 13 Prove that
sin ( ) tan tan
sin ( ) tan tan
x y x y
x y x y
+ +
=
− −
.
Solution We have
L.H.S.
sin ( ) sin cos cos sin
sin ( ) sin cos cos sin
x y x y x y
x y x y x y
+ +
= =
− −
Dividing the numerator and denominator by cos x cos y, we get
sin ( ) tan tan
sin ( ) tan tan
x y x y
x y x y
+ +
=
− −
.
Example 14 Show that
tan 3 x tan 2 x tan x = tan 3x – tan 2 x – tan x
Solution We know that 3x = 2x + x
Therefore, tan 3x = tan (2
x + x)
or
tan 2 tan
tan 3
1– tan 2 tan
x x
x
x x
+
=
or tan 3x – tan 3x tan 2x tan x = tan 2x + tan x
or tan 3x – tan 2x – tan x = tan 3x tan 2x tan x
or tan 3
x tan 2x tan x = tan 3x – tan 2x – tan x.
Example 15 Prove that
cos cos 2 cos
4 4
x x x
π π
+ + − =
Solution Using the Identity 20(i), we have
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72 MATHEMATICS
L.H.S.
cos cos
4 4
x x
π π
= + + −
( )
4 4 4 4
2cos cos
2 2
x x x – x
π π π π
+ + − + −
=
= 2 cos
4
π
cos x = 2 ×
1
2
cos x =
2
cos x = R.H.S.
Example 16 Prove that
cos 7 cos 5
cot
sin 7 – sin 5
x x
x
x x
+
=
Solution Using the Identities 20 (i) and 20 (iv), we get
L.H.S. =
7 5 7 5
2cos cos
2 2
7 5 7 5
2cos sin
2 2
x x x x
x x x x
+ −
+ −
=
cos
sin
cot
x
x
x=
= R.H.S.
Example 17 Prove that
sin 5 2sin 3 sin
tan
cos5 cos
x x x
x
x x
− +
= =
−
Solution We have
L.H.S.
sin 5 2sin3 sin
cos5 cos
x x x
x x
− +
=
−
sin 5 sin 2sin 3
cos5 cos
x x x
x x
+ −
=
−
2sin3 cos 2 2sin3
– 2sin3 sin 2
x x x
x x
−
=
sin 3 (cos2 1)
sin 3 sin 2
x x
–
x x
−
=
2
1 cos 2 2sin
sin 2 2sin cos
x x
x x x
−
= =
= tan x = R.H.S.
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TRIGONOMETRIC FUNCTIONS 73
EXERCISE 3.3
Prove that:
1. sin
2
π
6
+ cos
2
3
π
– tan
2
1
–
4 2
π
=
2. 2sin
2
6
π
+ cosec
2
2
7 3
cos
6 3 2
π π
=
3.
2 2
5
cot cosec 3tan 6
6 6 6
π π π
+ + =
4.
2 2 2
3
2sin 2cos 2sec 10
4 4 3
π π π
+ + =
5. Find the value of:
(i) sin 75° (ii) tan 15°
Prove the following:
6.
cos cos sin sin sin ( )
4 4 4 4
x y x y x y
π π π π
− − − − − = +
7.
2
π
tan
1 tan
4
π
1 tan
tan
4
x
x
x
x
+
+
=
−
−
8.
2
cos ( ) cos ( )
cot
sin ( ) cos
2
x x
x
x x
π + −
=
π
π − +
9.
3π 3π
cos cos (2
π ) cot cot (2π ) 1
2 2
x x x x
+ + − + + =
10. sin (n + 1)x sin (n + 2)x + cos (n + 1)x cos (n + 2)x = cos x
11.
3 3
cos cos 2 sin
4 4
x x x
π π
+ − − = −
12. sin
2
6x – sin
2
4x = sin 2x sin 10x 13. cos
2
2x – cos
2
6x = sin 4x sin 8x
14. sin2 x + 2 sin 4x + sin 6x = 4 cos
2
x sin 4x
15. cot 4x (sin 5x + sin 3x) = cot x (sin 5x – sin 3x)
16.
cos
cos
sin
sin
sin
cos
9
5
17
3
2
10
x
x
x
x
x
x
−
−
= −
17.
sin
sin
cos
cos
tan
5
3
5
3
4
x
x
x
x
x
+
+
=
18.
sin
sin
cos cos
tan
x
y
x y
x
y
−
+
=
−
2
19.
sin
sin
cos
cos
tan
x
x
x
x
x
+
+
=
3
3
2
20.
sin
sin
sin
cos
sin
x
x
x
x
x
−
−
=
3
2
2 2
21.
cos
cos
cos
sin
sin
sin
cot
4
3
2
4
3
2
3
x
x
x
x
x
x
x
+
+
+
+
=
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74 MATHEMATICS
22. cot x cot 2x – cot 2
x cot 3x – cot 3
x cot
x = 1
23.
2
2 4
4tan (1 tan )
tan 4
1 6 tan tan
x x
x
x x
−
=
− +
24. cos 4
x = 1 – 8sin
2
x cos
2
x
25. cos 6x = 32 cos
6
x – 48cos
4
x + 18 cos
2
x – 1
3.5 Trigonometric Equations
Equations involving trigonometric functions of a variable are called trigonometric
equations. In this Section, we shall find the solutions of such equations. We have
already learnt that the values of sin
x
and cos
x
repeat after an interval of 2π and the
values of tan
x
repeat after an interval of π. The solutions of a trigonometric equation
for which 0 ≤ x < 2π are called principal solutions. The expression involving integer
‘n’ which gives all solutions of a trigonometric equation is called the general solution.
We shall use ‘Z’ to denote the set of integers.
The following examples will be helpful in solving trigonometric equations:
Example 18 Find the principal solutions of the equation
sin x =
3
2
.
Solution We know that,
π 3
sin
3 2
=
and
2
π π π 3
sin sin π sin
3 3 3 2
= − = =
.
Therefore, principal solutions are
π
3
x
=
and
2
π
3
.
Example 19 Find the principal solutions of the equation
tan x = −
1
3
.
Solution We know that,
π 1
tan
6
3
=
. Thus,
π π 1
tan π – = – tan = –
6 6
3
and
π π 1
tan 2π tan
6 6
3
− = − = −
Thus
5
π 11π 1
tan tan
6 6
3
= = −
.
Therefore, principal solutions are
5
π
6
and
11
π
6
.
We will now find the general solutions of trigonometric equations. We have already
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TRIGONOMETRIC FUNCTIONS 75
seen that:
sin
x
=0 gives
x
= nπ, where n ∈ Z
cos
x
=0 gives
x
= (2n + 1)
π
2
, where n ∈ Z.
We shall now prove the following results:
Theorem 1 For any real numbers x and y,
sin x = sin y implies x = nπ + (–1)
n
y, where n ∈ Z
Proof If sin x = sin y, then
sin x – sin y = 0 or 2cos
x
y
x
y
+
−
2
2
sin
= 0
which gives cos
x
y
+
2
= 0 or sin
x
y
−
2
= 0
Therefore
x
y
+
2
= (2n + 1)
π
2
or
x
y
−
2
= nπ, where n ∈ Z
i.e. x = (2n + 1) π – y or x = 2nπ + y, where n∈Z
Hence x = (2n + 1)π + (–1)
2n + 1
y or x = 2nπ +(–1)
2n
y, where n ∈ Z.
Combining these two results, we get
x = nπ + (–1)
n
y, where n ∈ Z.
Theorem 2 For any real numbers x and y, cos x = cos y, implies
x
= 2nπ ± y,
where n ∈ Z
Proof If cos x = cos y, then
cos x – cos y = 0 i.e., –2 sin
x
y
+
2
sin
x
y
−
2
= 0
Thus sin
x
y
+
2
= 0 or sin
x
y
−
2
= 0
Therefore
x
y
+
2
= nπ or
x
y
−
2
= nπ, where n ∈ Z
i.e. x = 2nπ – y or x = 2nπ + y, where n ∈ Z
Hence x = 2nπ ± y, where n ∈ Z
Theorem 3 Prove that if x and y are not odd mulitple of
π
2
, then
tan x = tan y implies x = nπ + y, where n ∈ Z
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76 MATHEMATICS
Proof If tan x = tan y, then tan x – tan y = 0
or
sin
cos
cos
sin
cos cos
x
y
x
y
x y
−
= 0
which gives sin (x – y) = 0 (Why?)
Therefore x – y = nπ, i.e., x = nπ + y, where n ∈ Z
Example 20 Find the solution of sin
x
= –
3
2
.
Solution We have sin
x
= –
3
2
=
π π 4π
sin sin π sin
3 3 3
− = + =
Hence sin x =
4
π
sin
3
, which gives
4
π
π ( 1)
3
n
x n= + −
, where n ∈ Z.
A
Note
4
π
3
is one such value of x for which
3
sin
2
x = −
. One may take any
other value of x for which
sin x = −
3
2
. The solutions obtained will be the same
although these may apparently look different.
Example 21 Solve
cos x =
1
2
.
Solution We have,
1
π
cos cos
2 3
x = =
Therefore
π
2 π
3
x n
= ±
, where n ∈ Z.
Example 22 Solve
π
tan 2 cot
3
x x
= − +
.
Solution We have,
π
tan 2 cot
3
x x
= − +
=
π π
tan
2 3
x
+ +
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TRIGONOMETRIC FUNCTIONS 77
or
5
π
tan2 tan
6
x x
= +
Therefore
5
π
2 π
6
x n x= + +
, where n∈Z
or
5
π
π
6
x n= +
, where n∈Z.
Example 23 Solve sin 2x – sin 4x + sin 6x = 0.
Solution The equation can be written as
sin
sin
sin
6
2
4
0
x
x
x
+
−
=
or
2
4
2
4
0
sin
cos
sin
x
x
x
−
=
i.e.
sin
(
cos
)
4
2
2
1
0
x
x
−
=
Therefore sin 4x = 0 or
1
cos 2
2
x
=
i.e.
π
sin4 0 or cos 2 cos
3
x x= =
Hence
π
4 π or 2 2 π
3
x n x n
= = ±
, where n∈Z
i.e.
π π
or π
4 6
n
x x n
= = ±
, where n∈Z.
Example 24 Solve 2 cos
2
x + 3 sin x = 0
Solution The equation can be written as
(
)
2 1 3 0
2
− + =sin sinx x
or
2 3 2 0
2
sin sinx x− − =
or
(2sin 1) (sin 2) 0
x x
+ − =
Hence sin x =
1
2
−
or sin x = 2
But sin x = 2 is not possible (Why?)
Therefore sin x =
1
2
−
=
7
π
sin
6
.
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78 MATHEMATICS
Hence, the solution is given by
7
π
π ( 1)
6
n
x n= + −
, where n ∈ Z.
EXERCISE 3.4
Find the principal and general solutions of the following equations:
1.
tan x = 3
2. sec x = 2
3.
cot x = − 3
4. cosec x = – 2
Find the general solution for each of the following equations:
5. cos 4 x = cos 2
x
6. cos 3x + cos
x
– cos 2x = 0
7. sin 2x + cos
x
= 0 8. sec
2
2x = 1– tan 2x
9. sin x + sin 3x + sin 5x = 0
Miscellaneous Examples
Example 25 If sin x =
3
5
, cos y =
−
12
13
, where x and y both lie in second quadrant,
find the value of sin (x + y).
Solution We know that
sin (x + y) = sin x cos y + cos x sin y ... (1)
Now cos
2
x
= 1 – sin
2
x = 1 –
9
25
=
16
25
Therefore cos x =
±
4
5
.
Since x
lies in second quadrant, cos x is negative.
Hence cos x =
−
4
5
Now sin
2
y = 1 – cos
2
y = 1 –
144
169
25
169
=
i.e. sin y =
±
5
13
.
Since y lies in second quadrant, hence sin y is positive. Therefore, sin y =
5
13
. Substituting
the values of sin x, sin y, cos x and cos y in (1), we get
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TRIGONOMETRIC FUNCTIONS 79
sin( )x y+ = × −
+ −
×
3
5
12
13
4
5
5
13
=
36 20 56
65 65 65
− − = −
.
Example 26
Prove that
9 5
cos 2 cos cos 3 cos sin 5 sin
2 2 2
x x x
x x x− =
.
Solution We have
L.H.S. =
1 9
2cos 2 cos 2cos cos 3
2 2 2
x x
x x
−
=
1 9 9
cos 2 cos 2 cos 3 cos 3
2 2 2 2 2
x x x x
x x x x
+ + − − + − −
=
1
2
5
2
3
2
15
2
3
2
cos cos cos cos
x x x x
+ − −
=
1
2
5
2
15
2
cos cos
x x
−
=
5 15 5 15
1
2 2 2 2
2sin sin
2 2 2
x x x x
+ −
−
=
− −
=sin sin sin sin5
5
2
5
5
2
x
x
x
x
= R.H.S.
Example 27 Find the value of tan
π
8
.
Solution Let
π
8
x
=
. Then
π
2
4
x
=
.
Now
tan
tan
tan
2
2
1
2
x
x
x
=
−
or
2
π
2tan
π
8
tan
π
4
1 tan
8
=
−
Let y = tan
π
8
. Then 1 =
2
1
2
y
y
−
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80 MATHEMATICS
or y
2
+ 2y – 1 = 0
Therefore y =
− ±
= − ±
2 2 2
2
1 2
Since
π
8
lies in the first quadrant, y = tan
π
8
is positve. Hence
π
tan 2 1
8
= −
.
Example 28 If
3 3
π
tan = , π < <
4 2
x x
, find the value of sin
x
2
, cos
x
2
and tan
x
2
.
Solution Since
3
π
π
2
x< <
, cos
x
is negative.
Also
π 3π
2 2 4
x
< <
.
Therefore, sin
x
2
is positive and cos
x
2
is negative.
Now sec
2
x
= 1 + tan
2
x =
1
9
16
25
16
+ =
Therefore cos
2
x =
16
25
or cos x =
4
5
–
(Why?)
Now
2
2
2
sin
x
= 1 – cos x
=
1
4
5
9
5
+ =
.
Therefore sin
2
x
2
=
9
10
or sin
x
2
=
3
10
(Why?)
Again 2cos
2
x
2
= 1+ cos x
=
1
4
5
1
5
− =
Therefore cos
2
x
2
=
1
10
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TRIGONOMETRIC FUNCTIONS 81
or cos
x
2
=
−
1
10
(Why?)
Hence tan
x
2
=
sin
cos
x
x
2
2
3
10
10
1
= ×
−
= – 3.
Example 29 Prove that cos
2
x + cos
2
2
π π 3
cos
3 3 2
x x
+ + − =
.
Solution We have
L.H.S. =
2
π 2π
1 cos 2 1 cos 2
1 cos 2
3 3
2 2 2
x x
x
+ + + −
+
+ +
.
=
1 2
π 2π
3 cos 2 cos 2 cos 2
2 3 3
x x x
+ + + + −
=
1 2
π
3 cos 2 2cos 2 cos
2 3
x x
+ +
=
1
π
3 cos 2 2cos 2 cos π
2 3
x x
+ + −
=
1
π
3 cos 2 2cos 2 cos
2 3
x x
+ −
=
[ ]
1 3
3 cos 2 cos 2
2 2
x x
+ − =
= R.H.S.
Miscellaneous Exercise on Chapter 3
Prove that:
1.
0
13
π5
cos
13
π3
cos
13
π9
cos
13
π
cos2 =++
2. (sin 3x + sin x) sin x + (cos 3x – cos x) cos x = 0
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82 MATHEMATICS
3. (cos x + cos y)
2
+ (sin x – sin y)
2
= 4 cos
2
2
x y
+
4. (cos
x
– cos y)
2
+ (sin
x
– sin y)
2
= 4 sin
2
2
x y
−
5. sin x + sin 3x + sin 5x + sin 7x = 4 cos x cos 2x sin 4x
6.6.
6.
6.
6.
(sin 7 sin 5 ) (sin 9 sin 3 )
tan 6
(cos7 cos 5 ) (cos 9 cos 3 )
x x x x
x
x x x x
+ + +
=
+ + +
7. sin 3x + sin 2x – sin x = 4sin x cos
x
2
cos
3
2
x
Find sin
x
2
, cos
x
2
and tan
x
2
in each of the following :
8. tan
x
=
−
4
3
, x in quadrant II 9. cos x =
−
1
3
, x in quadrant III
10. sin
x
=
4
1
,
x
in quadrant II
Summary
®
If in a circle of radius r, an arc of length l
subtends an angle of
θ radians, then
l = r θ
®
Radian measure =
π
180
×
Degree measure
®
Degree measure =
180
π
×
Radian measure
®
cos
2
x + sin
2
x = 1
®
1 + tan
2
x = sec
2
x
®
1 + cot
2
x = cosec
2
x
®
cos (2nπ + x) = cos x
®
sin (2nπ
+ x) = sin x
®
sin (– x) = – sin x
®
cos (– x) = cos x
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TRIGONOMETRIC FUNCTIONS 83
®
cos (x + y) = cos x cos y – sin x sin y
®
cos (x – y) = cos x cos y + sin x sin y
®
cos (
π
2
x
−
) = sin x
®
sin (
π
2
x
−
) = cos x
®
sin (x
+ y) = sin x cos y + cos x sin y
®
sin (x
– y) = sin x cos y – cos x sin y
®
cos
π
+
2
x
= – sin x sin
π
+
2
x
= cos x
cos (π – x) = – cos x sin (π – x) = sin x
cos (π + x) = – cos x sin (π + x) = – sin x
cos (2π – x) = cos x sin (2π – x) = – sin x
®
If none of the angles x, y and (x
±
y) is an odd multiple of
π
2
, then
tan (x
+ y) =
tan
tan
tan tan
x
y
x y
+
−1
®
tan (x
– y) =
tan
tan
tan tan
x
y
x y
−
+1
®
If none of the angles x, y and (x
±
y) is a multiple of π, then
cot (x
+ y) =
cot cot 1
cot cot
x y
y x
−
+
®
cot (x
– y) =
cot cot 1
cot cot
x y
y x
−
+
®
cos 2x = cos
2
x – sin
2
x
= 2cos
2
x – 1 = 1 – 2 sin
2
x
2
2
1 tan
1 tan
– x
x
=
+
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84 MATHEMATICS
®
sin 2
x
= 2 sin
x
cos x
2
2 tan
1 tan
x
x
=
+
®
tan 2x =
2
2tan
1 tan
x
x
−
®
sin 3x
= 3sin
x
– 4sin
3
x
®
cos 3x = 4cos
3
x
– 3cos
x
®
tan 3x =
3
2
3tan tan
1 3tan
x x
x
−
−
®
(i) cos x + cos y = 2cos
cos
2 2
x y x y
+ −
(ii) cos x – cos y = – 2sin
sin
2 2
x y x y
+ −
(iii) sin x + sin y = 2 sin
cos
2 2
x y x y
+ −
(iv) sin x – sin y = 2cos
sin
2 2
x y x y
+ −
®
(i) 2cos x cos y = cos (
x
+ y) + cos (
x
– y)
(ii) – 2sin x sin y = cos (x + y) – cos (x – y)
(iii) 2sin x cos y = sin (x + y) + sin (x – y)
(iv) 2 cos x sin y = sin (x + y) – sin (x – y).
®
sin
x
= 0 gives x = nπ, where n ∈ Z.
®
cos x = 0 gives x = (2n + 1)
π
2
, where n ∈ Z.
®
sin x = sin y implies x = nπ + (– 1)
n
y, where n ∈ Z.
®
cos x = cos y, implies x = 2nπ ± y, where n ∈ Z.
®
tan x = tan y implies x = nπ + y, where n ∈ Z.
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TRIGONOMETRIC FUNCTIONS 85
Historical Note
T
he study of trigonometry was first started in India. The ancient Indian
Mathematicians, Aryabhatta (476), Brahmagupta (598), Bhaskara I (600) and
Bhaskara II (1114) got important results. All this knowledge first went from
India to middle-east and from there to Europe. The Greeks had also started the
study of trigonometry but their approach was so clumsy that when the Indian
approach became known, it was immediately adopted throughout the world.
In India, the predecessor of the modern trigonometric functions, known as
the sine of an angle, and the introduction of the sine function represents the main
contribution of the siddhantas (Sanskrit astronomical works) to the history of
mathematics.
Bhaskara I (about 600) gave formulae to find the values of sine functions
for angles more than 90°. A sixteenth century Malayalam work Yuktibhasa
(period) contains a proof for the expansion of sin (A + B). Exact expression for
sines or cosines of 18°, 36°, 54°, 72°, etc., are given by
Bhaskara II.
The symbols sin
–1
x, cos
–1
x, etc., for arc sin x, arc cos x, etc., were
suggested by the astronomer Sir John F.W. Hersehel (1813) The names of Thales
(about 600 B.C.) is invariably associated with height and distance problems. He
is credited with the determination of the height of a great pyramid in Egypt by
measuring shadows of the pyramid and an auxiliary staff (or gnomon) of known
height, and comparing the ratios:
H
S
h
s
=
= tan (sun’s altitude)
Thales is also said to have calculated the distance of a ship at sea through
the proportionality of sides of similar triangles. Problems on height and distance
using the similarity property are also found in ancient Indian works.
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