Which of the following is equivalent to $\cos 4x$?

Question

Accepted Answer

Using $\cos 2A = 2\cos^2 A - 1$, we have $\cos 4x = 2\cos^2(2x) - 1 = 2(2\cos^2 x - 1)^2 - 1 = 2(4\cos^4 x - 4\cos^2 x + 1) - 1 = 8\cos^4 x - 8\cos^2 x + 1$.

Trigonometric Functions

❓ Sibling Questions in this Chapter