1/x-1/y=2
同时乘以xy
y-x=2xy
同时乘以3
3y-3x=6xy
3x-3y=-6xy
(3x-2xy-3y)=-8xy
x-y=-2xy
(x-2xy-y)=-4xy
(3x-2xy-3y)除以(x-2xy-y)=2