Question

Cos 120
Cos Q = - cos (180-Q)

Answer 1

90 degree and 180 degree

180° – 60° = 120° ———– (1)

90° + 30° = 120° ———— (2)

Let’s use these now.

Cos 120° = cos(180° – 60°) = – cos 60° = -½ (since cos(180° – x) = – cos x)

Cos 120° = cos(90° + 30°) = – sin 30° = -½ (we know that cos (90° + x) = -sin x)