You can put this solution on YOUR website! Show that tan^2x - sin^2x=sin^4x(sec^2x)
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sin^2/cos^2 - sin^2x=sin^4x(sec^2x)
Divide by sin^2
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(1/cos^2) - 1 = sin^2x(sec^2x)
Multiply by cos^2
1 - cos^2 = sin^2
QED
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If someone can show an example where working one side only makes a difference I'd like to see it.