已知函数sin（70°+α）=1/3,则cos（2 - 知识问答

已知函数sin（70°+α）=1/3,则cos（2α-40°）=

3个回答

cos(2α-40)
=cos(40-2α)
=-cos[180-(40-2α)]
=-cos(140+2α)
=-cos[2(70+α)]
=-[1-2sin²(70+α)]
=-7/9

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