已知tanx/2=3,求cosx等于多少 - 知识问答

已知tanx/2=3,求cosx等于多少

1个回答

cosx = cos²(x/2)-sin²(x/2)
= {cos²(x/2)-sin²(x/2) } / { cos²(x/2)+sin²(x/2) }
= {1-tan²(x/2) } / { 1+tan²(x/2) }
= (1-3²)/(1+3²)
= -8/10
= -4/5

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