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cos^4x-sin^4x=cos2x 为什么
人气:298 ℃ 时间:2020-03-30 15:46:05
解答
cos^4x-sin^4x
=(cos^2x+sin^x)(cos^2x-sin^2x)
=1*(cos^2x-sin^2x)
=cos^2x-sin^2x
=cos2x
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