## walter9906 Group Title Find cos 2A, Sin A = -2/9 one year ago one year ago

You will need to use some identities here:$\bf \cos(2a)=1-2\sin^2(a)$We know what sin(a) is, so we just plug that in to the identity and evaluate.$\bf \cos(2a)=1-2(-2/9)^2=..?$Can you evaluate that? @walter9906
Thank you. I have one more. find Cos 2A $\cos A=\sqrt{3}/4$