三倍角公式推导,举个例子：
sin3a
=sin(2a+a)
=sin2acosa+cos2asina
=2sina(1-sin²a)+(1-2sin²a)sina
=3sina-4sin³a
sin2acosa+cos2asina
=2sina(1-sin²a)+(1-2sin²a)sina
就这两步之间怎么推？
人气：253 ℃ 时间：2020-08-22 06:39:16
解答
sin2acosa+cos2asina
=2sinacosa*cosa+(1-2sin²a)*sina (sin2a=2sinacosa cos2a=1-2sin²a)
=2sina(1-sin²a)+(1-2sin²a)sina
=3sina-4sin³a
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