已知x+y=1/2, xy=1 ,求x^3y+2x^2y^2+xy^3解：x^3y+2x^2y^2+xy^3 =(xy)x^2+2(xy)^2+(xy)y^2 =x^2+2+y^2 =(x+y)^2-2xy+2 =(1/2)^2 =1/4
