[กิตติ]

9.$\cos 2x=\frac{1}{8} ,\cot y=-\frac{3}{4} $

$\cos^2x=\frac{9}{16} \rightarrow \cos x=-\frac{3}{4} $
$\tan y=-\frac{4}{3}$
$\sin y=-\frac{4}{3}(\cos y)$
$\sin^2 y+\cos^2 y=1$
$\frac{16}{9}(\cos^2 y)+\cos^2 y=1$
$\cos^2 y=\frac{9}{25} $
$\cos y=\frac{3}{5}$

$\cos x+\cos y=-\frac{3}{4}+\frac{3}{5}=-\frac{3}{20} $