SOLVED IT. Sorry.
Determine an exact value for cos 13pi/12.
Since 13pi/12 is a degree of 195 degrees which is not related to a right angle triangle, I tried breaking it up into 3pi/4 + pi/3. Then I found the cos of both of those which got me:
cos(13pi/12) = cos(3pi/4) + cos(pi/3)
= (-1/sqr2) + (1/2)
I have no clue if i'm on the right track..
The answer: -1-sqr3 / 2sqr2