1/x-1/y=1xy/x-xy/y=xyy-x=xy∴3y-xy-3x/x-xy-y=3(y-x)-xy/(x-y)-xy =3xy-xy/-xy-xy =2xy/-2xy =-1
y-x=xy原式=【3xy-xy】/【-xy-xy】=2/[-2]=-1