(x+y)^2-（x^3+y^3）/（x+y）
=(x+y)^2-[(x+y)*(x^2-xy+y^2)]/(x+y) PS:x^3+y^3=(x+y)(x^2-xy+y^2)
=(x+y)^2-(x^2-xy+y^2)
=x^2+2xy+y^2-x^2+xy-y^2
=3xy