(2) ab(a+b)a5+b5=a+ba4+b4−ab(a2+b2)+a2b2=
ab(a−b)4+4ab(a2+b2)−6a2b2−ab(a2+b2)+a2b2⩾
ab3ab(a2+b2)−5a2b2=3(a2+b2)−5ab
Similarly, bc(b+c)b5+c5⩾3(b2+c2)−5bc,ca(c+a)c5+a5⩾3(c2+a2)−5ca, adding them up we get
ab(a+b)a5+b5+bc(b+c)b5+c5+ca(c+a)c5+a5⩾6−5(ab+bc+ca)
Since a2+b2+c2=1⩾ab+bc+ca, we have 6−5(ab+bc+ca)⩾3(ab+ bc+ca)−2. Therefore, (2) is a generalization of (1).