Question

Which identity is used for \(\cos^2 x\)?

Options

• \(\sin 3x = 3\sin x - 4\sin^3 x\)

• \(\cos^2 x = \frac{1-\cos 2x}{2}\)

• \(\cos 2x = 2\cos^2 x - 1\)

• \(\sin A\cos B = \frac{1}{2}[\sin(A+B)+\sin(A-B)]\)

Answer 1

The identity \(\cos 2x = 2\cos^2 x - 1\) directly contains \(\cos^2 x\). Rearranging it gives the useful even-power form.