Question

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

Answer 1

Prove trig expression

Explanation:

Apply 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
Because sin 180 = 0, cos 180 = -1, there for:
cos (180 - a) = - cos a