Question

Solve the following equations.
$1+2\cos 3x\cos x-\cos 2x=0.$

Answer 1

$1+2\cos 3x\cos x-\cos 2x=0$

$1+\cos 4x+\cos 2x-\cos 2x=0$

$\cos 4x=-1$

$4x=(2n+1)\pi$

$x=(2n+1)\frac{\pi }{4}.$