Asked by Breanna

prove the following identity
cos^4X - sin^4X = 1-2sin^2X
Answered by Reiny
LS = (cos^2x + sin^2x)(cos^2x - sin^2x)
= 1((1-sin^2x) - sin^2x)
= 1 - 2sin^2x
= RS
Answered by MathMate
Factorize the left hand side using:
A²-B²=(A+B)(A-B)
or
A<sup>4</sup>-B<sup>4</sup> = (A²+B²)(A²-B²)
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