Question

x y=5,xy=3求代数式x^3y-2x^2y^2=xy^3

Answer 1

xy=3 x+y=5 两边平方 x^2+2xy+y^2=25 x^2+y^2=25-2*3=19 x^3y-2x^2y^2+xy^3 =xy(x^2+y^2)-2(xy)^2 =3*19-2*3^2 =39