22. First, we have
(∑cycx)(∑cycx1)−9=∑cycxy(x−y)26(∑cycy+zx)−9=∑cyc(x+z)(y+z)3(x−y)2
From equations (7) and (8), to prove the original inequality, we only need to prove
cyc∑xy(x−y)2⩾cyc∑(x+z)(y+z)3(x−y)2
Let
Sx=yz1−(y+x)(z+x)3Sy=zx1−(z+y)(x+y)3Sz=xy1−(x+z)(y+z)3
Assume without loss of generality that x⩾y⩾z. Clearly, Sx⩾0. Since y+z⩾x, it is easy to see that xy+zy+y2−2xz⩾0, which means Sy⩾0. If Sz⩾0, the inequality is proved. If Sz⩽0, we can prove that Sy+Sz⩾0.
In fact,
=Sy+Sx=xz1−(x+y)(z+y)3+xy1−(x+z)(z+y)3xz(x+y)(z+y)y(x+z)−4yx2z+x2y2+2y2z2+y3x+y3z+x2z2+xz3+yz3
When x,y,z∈[1,2], the above expression is clearly greater than zero.
Since (x−z)2⩾(x−y)2, we have
cyc ∑Sx(y−z)2⩾Sx(y−z)2+(Sy+Sz)(x−y)2⩾0
From equation (10), we know that equation (9) holds, i.e., the original inequality holds, with equality if and only if x=y=z or x=2,y=z=1.