Question

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

Answer 1

Hint

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

Answer 2

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

Answer 3

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

Answer 4

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

Answer 5

@tkhunny