cos二倍角公式的推导余弦二倍角1.Cos2α=1 - 知识问答

cos二倍角公式的推导余弦二倍角1.Cos2α=1-2Sinα^22.Cos2a=2Cosa^2-1是如何推到出来的?

1个回答

Cos2α=cos(α+α)
=cosαcosα-sinαsinα
=cos^2α-sin^2α
=1-sin^2α-sin^2α
=1-2Sinα^2
Cos2α=cos(α+α)
=cosαcosα-sinαsinα
=cos^2α-sin^2α
=cos^2α-(1-cos^2α)
=2cos^2α-1

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