# Thread: Exact value for cos 13pi/12

1. ## Exact value for cos 13pi/12

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..

2. Originally Posted by kmjt

cos(13pi/12) = cos(3pi/4) + cos(pi/3)
Are you sure this is correct?

I believe

$\cos\left(\frac{13\pi}{12}\right) = \cos\left(\frac{3\pi}{4}+\frac{\pi}{3}\right) \neq \cos\left(\frac{3\pi}{4}\right)+\cos\left(\frac{\p i}{3}\right)$

3. Yes your right what I have in first post is wrong hehe. But I solved it using a compound angle formula.