tan(x+y)是怎么变成[tan(x)+tan(y)]/...

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tan(x+y)
=sin(x+y)/cos(x+y)
=(sinxcosy+cosxsiny)/(cosxcosy-sinxsiny)
=(tanx+tany)/(1-tanxtany) 分子分母同时除以cosxcosy.

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