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Posted by Breanna on Monday, September 28, 2009 at 4:22pm.

prove the following identity cos^4X - sin^4X = 1-2sin^2X

LS = (cos^2x + sin^2x)(cos^2x - sin^2x) = 1((1-sin^2x) - sin^2x) = 1 - 2sin^2x = RS

Factorize the left hand side using: A²-B²=(A+B)(A-B) or A^{4}-B^{4} = (A²+B²)(A²-B²)

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