证明1-2sinxcosx/cos^2x-sin^2x=1-tanx/1+tanx

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，先把TANX化成SINX/COSX,等式右边上下同乘COSX(COSX-SINX)即得左边

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(1-2sinxcosx)/(cos^2x-sin^2x)
=(cosx-sinx)^2/[(cosx-sinx)*(cosx+sinx)]
=(cosx-sinx)/丛隐或渗伍携吵(cosx+sinx)
=(1-tanx)/(1+tanx)

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