设tan(θ/2)=t,求证：sinθ=2t/(1+t^2),cosθ=(1-t^2)/(1+t^2),tanθ=2t/(1-t^2)
sec没有学过
人气：449 ℃ 时间：2020-02-16 17:49:06
解答
下面sin,cos均指sin(θ/2),cos(θ/2)
sinθ=2sincos/(cos^2+sin^2) (上下同除cos^2)
=2t/(1+t^2)
cosθ=(cos^2-sin^2)/(cos^2+sin^2)
=(1-t^2)/(1+t^2)
tanθ=sinθ/cosθ=2t/(1-t^2)
万能公式的证明而已……
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