sinx/cosx=tanx=x
sinx=xcosx
同理
siny=ycosy
所以原式=(sinxcosy-cosxsiny)/(x-y)-(sinxcosy+cosxsiny)/(x+y)
=(xcosxcosy-ycosxcosy)/(x-y)-(xcosxcosy+ycosxcosy)/(x+y)
=cosxcosy(x-y)/(x-y)-cosxcosy(x+y)/(x+y)
=cosxcosy-cosxcosy
=0