Question

If cos θ = \(\frac {3}{4}\) then value of cos 2θ is ___________

(a) \(\frac {1}{6}\)

(b) \(\frac {1}{4}\)

(c) \(\frac {1}{8}\)

(d) \(\frac {3}{8}\)

This question was addressed to me in semester exam.

My doubt is from Trigonometric Ratios in chapter Trigonometry of Mathematics – Class 10

Answer 1

Right option is (c) \(\frac {1}{8}\)

Best explanation: cos 2θ = 2cos θ^2 – 1

cos θ = \(\frac {3}{4}\)

cos 2θ = 2(\(\frac {3}{4}\))^2 – 1

= \(\frac {1}{8}\)