原式 =x(x+y)(x-y-x-y) =-2xy(x+y)代入原式= -2*6*5=-30
=x(x+y) *(x-y)-x(x+y)^2=x(x+y)*[(x-y)-(x+y)] 提取公因式x(x+y)=x(x+y)*(-2y) 合并=-2xy(x+y)=-12*5=-60
