Question

Evaluate cos 510 cos 330 °​

Answer 1

cos510 can be written as

cos(360 × 1 + 150) =cos150

now, cos150 = cos(90 + 60) = -sin60°

cos330° can be written ascos(360° × 1 - 30°) = cos30°

now,

-sin60°.cos30°

= -√3/2 . √3/2

= -3/4