Prove: $\dfrac{cos2A}{2}+1=\dfrac{-cosA}{2}+\dfrac{1}{4}$
Please, help

Question

Prove: $\dfrac{cos2A}{2}+1=\dfrac{-cosA}{2}+\dfrac{1}{4}$
Please, help

rational · Answer

Hint 

cos(2A) = cos(A + A) = cosAcosA - sinAsinA = cos^2A - sin^2A = 2cos^2A-1

loser66 · Answer

Can you give me more. I am sorry for being blank right now.

rational · Answer

I don't have pen and paper handy... i would just mess with that angle sum identity
 but ur equation/identity doesn't look correct hmm

loser66 · Answer

Another question relates on it: 
Let y = 2pi/5
2y = 4pi/5
$cos 2y = 2 cos^2 y -1$
then $cos^2y -\dfrac{1}{2} cos2y-\dfrac{1}{2}=0$
How to write it w.r.t y only. I know the answer is $cos^2y+1/2 cos y -1/4 =0$ but don't know how to do

loser66 · Answer

@tkhunny