XY除于X加Y等于3,则XY=3(X+Y)
2X-3XY+2Y的和除于-X+3XY-Y
=2X-9(X+Y)+2Y的和除于-X+9(X+Y)-Y
=-7(X+Y)÷8(X+Y)
=-7/8是除于X+Y的和是啊，没错为什么是9(X+Y)因为XY=3(X+Y)，所以3XY=9（X+Y）那为什么-X+9(X+Y)-Y会等于8(X+Y)呢？-X+9(X+Y)-Y=-X-Y+9(X+Y)=-(X+Y)+9(X+Y)=8(X+Y)