Question

How do you prove cos(180°+a) = -cos(a)?

Answer 1

Prove trig identity.

Explanation:

Use the trig identity:
cos (a + b) = cos a.cos b - sin a.sin b
cos (180 + a) = cos 180.cos a - sin 180.sin a
Trig table -->
sin 180 = 0, and cos 180 = -1
Therefor,
cos (180 + a) = - cos a