I have no idea how I have messed up this identity proof question
Prove: Cos4x is identical to 8cos^(4)x-8cos^(2)x+1
I started with the left hand side, rewriting it as
I then used the compound formula to obtain
Using double angle formulae
(cos^(2)x-sin^(2)x)^(2) - (2sinxcosx)^2
Expanding the squared brackets to obtain
Collecting like terms
Using the identity sin^(2)x = 1-cos^(2)x and expanding the bracket
Collecting like terms again
Using the identity sin^(4)x = (1-cos^(2)x)^(2)
Collecting like terms for the last time cancels out the trig terms, leaving just...
I know this is a lot of workings, but can somebody please tell me where I've gone wrong?
C3 Identify proof question
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