# Mathematics > Trigonometry

• If in a circle of radius r, an arc of length l subtends and angle of  radians, then l = r
• Radian measure =(/180)× Degree measure
• Degree measure =(180/)× Radian measure
• cos2 x + sin2 x = 1
• 1 + tan2 x = sec2 x
• 1 + cot2 x = cosec2 x
• cos (2nπ + x) = cos x
• sin (2nπ + x) = sin x
• sin (– x) = – sin x
• cos (– x) = cos x
• cos (x + y) = cos x cos y – sin x sin y
• cos (x – y) = cos x cos y + sin x sin y
• cos (ð/2− x ) = sin x
• sin (ð/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 – 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
• cos (π/2+ x ) = -sin x
• sin (π/2+ x ) = cos x
• cos 2x = cos2 x – sin2 x = 2cos2 x – 1 = 1 – 2 sin2 x = (1 – tan2x)/ (1 + tan2x)
• tan(x + y) =  (tan x + tan y)/(1 – tan x tan y)
• cot(x + y) = (cot x cot y – 1)/(cot x + cot y)

### Examples

#### Question

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.

#### Question

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 (2x + x)

tan 3x = (tan 2x +tan x)/(1 – tan x tan 2x)

tan 3x – tan 3x tan 2x tan x = tan 2x + tan x

tan 3x – tan 2x – tan x = tan 3x tan 2x tan x

tan 3x tan 2x tan x = tan 3x – tan 2x – tan x.

#### Question

sin 2x – sin4 x + sin 6x = 0.

#### Solution

The equation can be written as

sin 6x + sin 2x − sin 4x = 0

2 sin 4x cos2x − sin 4x = 0

i.e. sin 4x(2 cos2x − 1) = 0

sin 4x = 0 or cos 2x = 1/2

sin4x = 0 or cos 2x = cos π/3

4x = nπ or 2nπ ± π/3