tan(x+y)tan(x-y)=sin^2x-sin^2y/cos^2x-sin^2y 顺便问一下. tan,sin,cos是什么意思?

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tan(x+y)tan(x-y)=sin^2x-sin^2y/cos^2x-sin^2y 顺便问一下. tan,sin,cos是什么意思?

tan(x+y)tan(x-y)=sin^2x-sin^2y/cos^2x-sin^2y 顺便问一下. tan,sin,cos是什么意思?
tan(x+y)tan(x-y)=sin^2x-sin^2y/cos^2x-sin^2y 顺便问一下. tan,sin,cos是什么意思?

tan(x+y)tan(x-y)=sin^2x-sin^2y/cos^2x-sin^2y 顺便问一下. tan,sin,cos是什么意思?
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]