tan(x+y)tan(x-y)=sin^2x-s - 知识问答

tan(x+y)tan(x-y)=sin^2x-sin^2y/cos^2x-sin^2y 顺便问一下. tan,sin,

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tan,正切；sin,正弦；cos,余弦
tan(x+y)tan(x-y)
=sin(x+y)/cos(x+y) * sin(x-y)/cos(x-y)
=sin(x+y)sin(x-y) / [cos(x+y)cos(x-y)]
=(sinxcosy+cosxsiny)(sinxcosy-cosxsiny) / [(cosxcosy-sinxsiny)(cosxcosy+sinxsiny)]
=[(sinxcosy)^2-(cosxsiny)^2] / [(cosxcosy)^2-(sinxsiny)^2]

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