56. Let a,b,c be positive real numbers, prove: ba+cb+ac≥3ab+bc+caa2+b2+c2 (Vo Quoc Ba Can)
Solution
Proof: Note that, if a≥b≥c, then (ba+cb+ac)−(ab+bc+ca)=abc(a−b)(a−c)(c−b)≤0
Therefore, we only need to consider the case a≥b≥c. Squaring both sides, we have cyc∑b2a2+cyc∑a2b≥ab+bc+ca9(a2+b2+c2)
Additionally, using the following identities ab+bc+ca−3=bc(b−c)2+ac(a−b)(a−c)b2a2+c2b2+a2c2−3=b2c2(b−c)2(b+c)+a2b2(a2−b2)(a2−c2)a2+b2+c2−(ab+bc+ca)=(b−c)2+(a−b)(a−c)
The inequality is equivalent to (b−c)2M+(a−b)(a−c)N≥0
where M=bc2+b2c2(b+c)2−ab+bc+ca9; N=ac2+a2b2(a+b)(a+c)−ab+bc+ca9
If b−c≥a−b, then 2(b−c)2≥(a−b)(a−c). Thus, we have M≥bc6−ab+bc+ca9≥0M+2N≥bc6−ab+bc+ca18≥0
Therefore, we have (b−c)2M+(a−b)(a−c)N≥21(a−b)(a−c)(M+2N)≥0 .
Otherwise, if b−c≤a−b, then 2b≤a+c, of course, M≥0 and N≥ac2+ab2a+b+c≥ac2+ab3≥ac+ab(2+3)2>ab+bc+ca9
Equality holds if and only if a=b=c.
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