Find the distance from the eye at which a coin of $2\ \text{cm}$ diameter should be held so as to conceal the full moon whose angular diameter is $31'$.
$\sqrt{1+\cos 2x}=\sqrt{2},|\cos x|$. On $\left[\frac{\pi}{2},\pi\right]$, $\cos x\le 0$ so $|\cos x|=-\cos x$.
LHS $=\sqrt{2},|\cos x|$, RHS $=\sqrt{2},|\cos x|$ only if $\cos^{-1}(\cos x)=|\cos x|$, which never holds for $x\in\left[\frac{\pi}{2},\pi\right]$.
Hence no real solution.