(x+y)(x^3+y^3)
=(x+y)^2(x^2-xy+y^2)
=(x+y)^2[(x-y/2)^2+3y^2/4]
因为(x-y/2)^2>=0,3y^2/4>=0
所以(x-y/2)^2+3y^2/4>=0
又(x+y)^2>=0
所以(x+y)(x^3+y^3)>=0