Answer
$$\cos195^\circ=\frac{-\sqrt2-\sqrt6}{4}$$
Work Step by Step
In Exercise 71, we have proved that $$\cos195^\circ=-\cos15^\circ$$
In Exercise 72, we found that $$-\cos15^\circ=\frac{-\sqrt2-\sqrt6}{4}$$
Therefore, $$\cos195^\circ=\frac{-\sqrt2-\sqrt6}{4}$$