Hiii~

I don't understand the double angle identity~ Can anyone explain to me? :3

So i'm given

cos81sin81

and i don't know what to do with it. I have the answers and it says it's 1/2sin162.

I don't understand how you get the answer D;

Annd how do you do

2cos^2 3.4-1

the answer to that one is cos6.8

I don't get it at all. ); can anyone explain?

thaanks :]