Answer
$-\frac{\sqrt3+1}{2\sqrt2}$
Work Step by Step
Using the given formula and the known values: $\cos195^\circ=\cos60^\circ\cos135^\circ-\sin60^\circ\sin135^\circ=\frac{1}{2}(-\frac{1}{\sqrt2})-\frac{\sqrt3}{2}\frac{1}{\sqrt2}=-\frac{\sqrt3+1}{2\sqrt2}$
