Question

Find the values of the following trigonometric ratio.

cos (-210°)

Answer 1

cos(-210°) = cos 210° (∵ cos(-θ) = cos θ)

[∵ 180 + 30°. No change in T-ratio. 210° lies 3rd quadrant ‘cos’ is negative]

= cos(180° + 30°)

= - cos 30°

`= - sqrt3/2`