Question

verify the Identity

cos4u=cos^(2)(2u) - sin^(2)(2u)

Answer 1

compare with standard identity

cos(2x) = cos^2(x) - sin^2(x)

Answer 2

replace x with 2u

Answer 3

@cwrw238 so what is the answer?

Answer 4

Oh I thought those were 22's lolol, I'm glad you fixed it ambition :D I was really scratching my head over this one earlier.

Answer 5

As penguin guy said, this is the Double Angle Formula for Cosine.
\[\huge \cos (2u)=\cos^2u-\sin^2u\]
Let's apply this to our problem,\[\large \cos(4u) \quad = \quad \cos(2\cdot 2u)\]\[\large \cos(2\cdot 2u) \quad = \quad \cos^2(2u)-\sin^2(2u)\]

Answer 6

ok so is there something else i have to do?

Answer 7

@zeepdrix

Answer 8

@zepdrix

Answer 9

no - xepdrix as done it all for you