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)):}`