Please Help!
Use a double-angle formula to find the exact value of the given expression.
2 cos^2(11pi/12) -1

Question

Please Help! 
Use a double-angle formula to find the exact value of the given expression.
2 cos^2(11pi/12) -1

tkhunny · Answer

Alarms should go off in your head.  This is why we have you do endless identities.  You have to see them when you encounter them!  This one even TELLS you to use the double angle formula.  Do it!

$\cos(2x) = 2\cos^{2}(x) - 1$

Go!!!

anonymous · Answer

I have no idea how to even begin that problem =(

tkhunny · Answer

Hogwash.

1) You wrote it down. That was a beginning. See, you do have an idea.
2) The problem statement tells you how to proceed. You must, therefore, have an idea.
3) I gave you the required formula. This is another step in the right direction.

So, there you go. You are already three steps in.
Simply apply the Double Angle formula and you'll be done.

You have x = 11pi/12. Substitute away!