SOLVED IT. Sorry.

Question:

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