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To show it's an identity:
(1-cos^2x)(1-tan^2x) = (sin^2x-2sin^4x) divided by 1-sin^2x
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sin^2*(1-tan^2) = (sin^2-2sin^4)/cos^2
Multiply by cos^2
sin^2*(cos^2 - sin^2) = sin^2 - 2sin^4
sin^2cos^2 - sin^4 = sin^2 - 2sin^4
sin^2cos^2 = sin^2 - sin^4
sin^2cos^2 = sin^2*(1 - sin^2)
sin^2cos^2 = sin^2cos^2