（x+3y)(x-2y)-(x-y)(x+y)=（x+y+2y）(x-y-y)-(x-y)(x+y)=[(x-y)(x+y)-(x+y)*y+2y(x-y)-2y ^2] -(x-y)(x+y)=-(x+y)*y+2y(x-y)-2y ^2=-xy-y^2 +2xy-2y^2-2y^2=xy-5y^2 =y(x-5y)