Hi,

I'm fairly new to more complex trigonometry, and i've found myself stumped on a certain question which can be found in exercise 6C of the Cambridgbe Advanced Mathematics course textbook. The question reads as follows...

If Sin A = 2/3, and A is obtuse, find the

**exact values** of Cos A, Sin 2A and tan 2A.

I know how to use the double angle formula to calculate Sin 2A and tan 2A, but i'm stuck on trying to find Cos A which i need to put into the double angle formula to find the last two parts...

Any ideas? Thanks