证明：[tan(x+y)-tanx]/1+tanxtan(x+y)=sin2x/2cosy^2

证明：[tan(x+y)-tanx]/1+tanxtan(x+y)=sin2x/2cosy^2
证明：[tan(x+y)-tanx]/1+tanxtan(x+y)=sin2x/2cosy^2

证明：[tan(x+y)-tanx]/1+tanxtan(x+y)=sin2x/2cosy^2
左边=tan(x+y-x)=tany
右边=2sinycosy/2cos^2y
=siny/cosy
=tany
左边=右边
[tan(x+y)-tanx]/1+tanxtan(x+y)=sin2y/2cosy^2

啥东东啊？