Which is the value of the sum cos 75 + cos 15?

cos75 + cos15 =
?

We know that:

cosx + cosy = 2cos(x+y)/2 *
cos(x-y)/2

Then :

cos75+cos15= 2 cos(75+15)/2 *
cos(75-15)/2

                  = 2cos(90/2) * cos(60/2)

                   = 2cos(45)*cos(30)

                  =
2(sqrt2)/2) * sqrt(3)/2

                  = sqrt(6) /2