xy(x+y)²-4x³y³=xy[(x+y)²-(2xy)²]=xy(x+y+2xy)(x+y-2xy) 如果不懂，请追问，祝学习愉快！
xy(x+y)^2-4x^3y^3=xy(x+y)^2-xy*(4x^2y^2)=xy[(x+y)^2-(4x^2y^2)]=xy(x+y-2xy)(x+y+2xy)
