The proof of inequality (AC + BD) & sup2; ≤ (A & sup2; + B & sup2;) (C & sup2; + D & sup2;)

The proof of inequality (AC + BD) & sup2; ≤ (A & sup2; + B & sup2;) (C & sup2; + D & sup2;)

(A & sup2; + B & sup2;) (C & sup2; + D & sup2;) - (AC + BD) & sup2; = (a ^ 2 * C ^ 2 + A ^ 2 * d ^ 2 + B ^ 2 * d ^ 2) - (a ^ 2 * C ^ 2 + 2acbd + B ^ 2 * d ^ 2) = a ^ 2 * d ^ 2-2ad * BC + B ^ 2 * C ^ 2 = (AD BC) ^ 2 > = 0, so: (AC + BD) & sup2; ≤ (A & sup2; + B & sup2;) (C & sup2; + D & sup2;)