trig
posted by Steve .
Use a halfangle formula to find the exact value of cos 5ð/12

your symbol for pi did not come out, just type pi, that seems to be the simplest way.
A lot of students seem to find it easier to think in degrees,
5pi/12 = 75º
cos(5pi/12) radians
= cos 75º
= cos(45 + 30)
= cos45cos30  sin45sin30
= (√2/2)(√3/2)  √2/2)(1/2)
= (√6  √2)/4