Question

Solve the following equation:
$1+\cos x+\cos 2x=0$

Answer 1

$1+\cos x+{2\cos }^{2}x-1=0$

$\cos x(1+2\cos x)=0$

Either, $\cos x=0,\$

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

or, $\cos x=-\frac{1}{2}$

$x=2n\pi \pm \frac{2\pi }{3}$