Question

Prove that cos80 + cos40 = cos20​

Answer 1

Step-by-step explanation:

cos 80 + cos 40

2 cos 80+40/2 cos 80-40/2. (we have a formula cos c +cos d = 2 cos c+d/2 cos c-d/2)

2 cos 120/2 cos 40/2

2 cos 60 cos 20

2 × 1/2 cos 20

cos 20