Answer
$x=\{\pi\}$
Work Step by Step
$cos(2x)-cos(x)-2=0$
$2cos^2(x)-1-cos(x)-2=0\;\;\;\;\;\;\;\;\;\;$
$2cos^2(x)-cos(x)-3=0\;\;\;\;\;\;\;\;\;\;\;\;$
$cos(x)=\frac{-(-1)\pm \sqrt{(-1)^2-(4.2.(-3))}}{2.2}=-1,\frac{3}{2}$
$cos(x)=-1$
$\theta= cos^{-1}(-1)$
$x=\pi\;\;\;\;\;\;\;\;\;\;\;\;\;\;$
$cos(\theta)=\frac{3}{2}$
$x=$No solution because cosine line between 1 and -1
$x=\{\pi\}$
