证明:cos^4 a-sin^4 a=cos^2 a(1-tan a)(1+ tan a)
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证明:cos^4 a-sin^4 a=cos^2 a(1-tan a)(1+ tan a)
证明:cos^4 a-sin^4 a=cos^2 a(1-tan a)(1+ tan a)
证明:cos^4 a-sin^4 a=cos^2 a(1-tan a)(1+ tan a)
cos^4 a-sin^4 a
=(cos^2 a+sin^2 a)(cos^2 a-sin^2 a)
=(cosa+sina)(cosa-sina)
=(cosa+sina)(cosa-sina)
=cos^2 a(1-sina/cosa)(1+ sina/cosa)
=cos^2 a(1-tan a)(1+ tan a)
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