Good morning all!

There is a problem that I am unable to solve:

Find the exact value of cos[3(pi)/8].

I can't figure out how to solve that for exact solution. My teacher said to use a double-angle identity but I am very confused. My class is new to all this. So any help regarding this problem would be appreciated. Also if it isn't asking to much, can you show the steps if you are able to help me.

Thanks, Anth :)