Once again, we use the 30°-60° and 45°-45° triangles to find the exact value.

`cos 15^("o")=cos(60^("o")-45^("o"))`

`=cos 60^("o")\ cos 45^("o")+sin\ 60^("o")\ sin\ 45^("o")`

`=1/2(1)/sqrt2+sqrt3/2(1)/sqrt2`

`=(1+sqrt3)/(2sqrt2)`