1/x-1/y=5(y-x)/xy=5(x-y)/xy=-5(3x+5xy-3y)/(x-3xy-y)=[3(x-y)+5xy]/[(x-y)-3xy]=[3(x-y)/xy+5]/[(x-y)/xy-3]=[3×(-5)+5]/(-5-3)=(-10)/(-8)=5/4
