It is proved that for any ABCD, there is always (AC + BD) & sup2; ≤ (A & sup2; + B & sup2;) (C & sup2; + D & sup2;)

It is proved that for any ABCD, there is always (AC + BD) & sup2; ≤ (A & sup2; + B & sup2;) (C & sup2; + D & sup2;)

(A & sup2; + B & sup2;) (C & sup2; + D & sup2;) = A & sup2; C & sup2; + A & sup2; D & sup2; + B & sup2; D & sup2; = (A & sup2; D & sup2; + B & sup2; C & sup2;) + (A & sup2; C & sup2; + B & sup2; D & sup2;) ≥ 2 radical (A & sup2; D & sup2; B & sup2; C & sup2;) + (A & S