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Answer: By using the formula, Cos(a+b) = cos a cos b – sin a sin b, we will find the value of Cos 135°
Cos 135°= cos(90°+45°)
Let us assume that a = 90° and b = 45°
Cos 135° = (cos 90° cos 45°) – (sin 90° sin 45°)
{As we know that ,Cos 90°=0
Cos 45°=1/√2
Sin 90°=1
Sin 45°=1/√2}
cos 135°= (0 x 1/√2) – (1 x 1/√2)
cos 135°= –1/√2
The quadrants must be remembered whenever trigonometric computations are performed. The signs are quite important in identifying the proper response.
