证明 sin^4a+cos^4a=1-2sin^2a乘以cos^2a
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证明 sin^4a+cos^4a=1-2sin^2a乘以cos^2a
证明 sin^4a+cos^4a=1-2sin^2a乘以cos^2a
证明 sin^4a+cos^4a=1-2sin^2a乘以cos^2a
(sina^2+cosa^2)^2=sina^4+cosa^4+2sina^2cosa^2=1,即sina^4+cosa^4=1-2sina^2cosa^2
sin^4a+cos^4a=(sin^2a+cos^2a)^2-2sin^2acos^2a= 1-2sin^2acos^2a
(sina)^2+(cosa)^2=1
[(sina)^2+(cosa)^2]^2=1=(sina)^4+(cosa)^4+2(sina)^2(cosa)^2
(sina)^4+(cosa)^4=1-2(sina)^2(cosa)^2
sin^4a+cos^4a=sin^4a+cos^4a+2sin^2acos^2a-2sin^2acos^2a
=(sin^2a+cos^2a)^2-2sin^2acos^2a
=1-2sin^2a cos^2a
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证明 sin^4a+cos^4a=1-2sin^2a乘以cos^2a
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