sin^4a-cos^4a=sin^2a-cos^2a

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sin^4a-cos^4a=sin^2a-cos^2a
sin^4a-cos^4a=sin^2a-cos^2a

sin^4a-cos^4a=sin^2a-cos^2a
sin^4a-cos^4a=(sin^2a-cos^2a)(sin^2a+cos^2a)=(sin^2a-cos^2a)*1=sin^2a-cos^2a