Question

Find the value of the given trigonometric function.
cos 420°

Answer 1

Recall that cos(x + y) = cos(x) cos(y) - sin(x) sin(y). So

cos(420°) = cos(360° + 60°)

cos(420°) = cos(360°) cos(60°) - sin(360°) sin(60°)

cos(420°) = cos(60°)

since cos(360°) = 1 and sin(360°) = 0. Then

cos(420°) = cos(60°) = 1/2