If cosθ = √3/2, what is cos2θ?

Think it through first, then pick an option.

Answer and explanation

Correct answer: b 1/2

cosθ = √3/2 ⇒ θ = 30°; cos2θ = cos60° = 1/2.

Why the other options are wrong

a 0: 0 would need 2θ = 90°, that is θ = 45°; it is a different quantity from the one asked for.
c √3/2: √3/2 is cosθ itself, not the value at the doubled angle.
d 1: 1 would need 2θ = 0°; it is a different quantity from the one asked for.