Question

(a) Show that cos A + cos(120° + A) + cos(120° - A) = 0.​

Answer 1

Explanation:

Using the two trig identities (shown below)

= cos A + cos 120 cos A - sin 120 sinA + cos 120 cos A + sin120 sinA

= cos A + 2 cos 120 cos A cos 120 = -1/2

= cos A + 2 ( -1/2) cosA

= cos A - cos A

= 0