AlgebraDifficulty 7.2National olympiad, round 2Prove it
38. Let a,b,c be non-negative real numbers, prove: b2+c2a2+2bc+c2+a2b2+2ca+a2+b2c2+2ab≥3 (Vo Quoc Ba Can, Vu Dinh Quy)
Solution
To prove: Without loss of generality, assume a≥b≥c. First, we will prove b2+c2a2+c2+c2+a2b2+c2≥ba+ab
In fact, this inequality is equivalent to b2+c2a2+c2+c2+a2b2+c2≥ba+ab⇔ab(c2+a2)(b2+c2)(a−b)2(a+b)(ab−c)2≥0
This is true because a≥b≥c. Using this result, we have b2+c2a2+2bc+c2+a2b2+2ca+a2+b2c2+2ab≥b2+c2c2+a2+c2+a2b2+c2+a2+b22ab≥ba+ab+a2+b22ab
Let x=ba+ab≥2. If x≥3, the inequality is obviously true. Otherwise, assume x≤3, we only need to prove ba+ab+a2+b22ab=x+x2−22≥3
Since x2−22−(3−x)2=x2−3(x−2)2(−x2+2x+5)≥0
Equality holds if and only if a=b,c=0 and their permutations.
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