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求证sin^4a-cos^4a=-cos2a
如题 求证 sin^4(a)-cos^4(a)=-cos(2a)
人气:151 ℃ 时间:2020-05-30 20:41:44
解答
sin^4(a)-cos^4(a)
=(sin^2a+cos^2a)(sin^2a-cos^2a)
=-(cos^2a-sin^2a)
=-cos(2a)
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