Question

Express the following as a sum or difference of two trigonometric function:

2 cos 35° cos 75°

Answer 1

2 cos 35° cos 75°

= cos(35° + 75°) + (35° – 75°)

= cos 110° + cos (– 40)°

= cos 110° + cos 40° ...[∵ cos(– θ) = cos θ]