Use sum and difference identitites to find the exact value of co 15 degrees

Answer 1

Cos(15)=cos(45-30)=cos45cos30-sin45sin30

you should have committed all those to memory.

Answer 2

15 = 45-30
cos15 = cos45cos30 + sin45sin30
= 1/√2 * √3/2 + 1/√2 * 1/2
= (√3-1) / 2√2

Answer 3

oops that's (√3+1) / 2√2