xy+y^2+x-y-2=(xy+x)+(y^2-y-2)=x(y+1)+(y-2)(y+1)=(y+1)(x+y-2)
xy+y^2+x-y-2=(x+y-2)(y+1)
原式=x(y+1)+(y^2-y-2)=x(y+1)+(y-2)(y+1)=(y+1)(x+y-2)
