Question

Find cos 2x if cos x = 1/\(\sqrt{2}\).

(a) 1/2

(b) 0

(c) \(\sqrt{3}\)/2

(d) 1

The question was posed to me during an interview.

My question comes from Trigonometric Functions of Sum and Difference of Two Angles-1 topic in chapter Trigonometric Functions of Mathematics – Class 11

Answer 1

Correct answer is (b) 0

Easy explanation: We know, cos 2x = cos^2x – sin^2x = 2cos^2x – 1 {sin^2x = 1-cos^2x}

= 2(1/\(\sqrt{2}\))^2-1 = 2(1/2) – 1 = 1-1 = 0.