Explain how to evaluate cospi/12.

We know that cos (pi/6) = (sqrt
3)/2

cos (pi/6) = cos 2*(pi/12) = 2*[cos (pi/12)]^2 -
1

=> (sqrt 3)/2 = 2*[cos (pi/12)]^2 -
1

=> 2*[cos (pi/12)]^2 = 1 + (sqrt
3)/2

=> [cos (pi/12)]^2 = 1/2 + (sqrt
3)/4

=> cos (pi/12) = sqrt [ 1/2 + (sqrt
3)/4]

The value of cos (pi/12) = sqrt [ 1/2 +
(sqrt 3)/4]