要求tanx/tany则x,y≠π/2+kπ,且y≠kπ(k为整数）,则cosxcosy≠0
sin(x+y)/cos(x-y)=m/n
(sinxcosy+cosxsiny)/(cosxcosy+sinxsiny)=m/n
(tanx+tany)/(1+tanxtany)=m/n(等式左边同除以cosxcosy)