AlgebraDifficulty 7.5National olympiad, round 2Prove it
45. Let a,b,c be positive real numbers, prove that (2+ba)2+(2+cb)2+(2+ac)2≥ab+bc+ca9(a+b+c)2 (Pham Kim Hung)
Solution
Prove that the inequality is equivalent to cyc∑b2a2+4cyc∑ba≥ab+bc+ca9(a2+b2+c2)2+6
Consider the following identities: ba+cb+ac−3=ab(a−b)2+ac(c−a)(c−b)a2+b2+c2−ab−bc−ca=(a−b)2+(b−c)2+(c−a)2
The inequality becomes the following form: (a−b)2M+(c−a)(c−b)N≥0
where M=ab4+a2b2(a+b)2−ab+bc+ca9,N=ac4+a2c2(c+a)(c+b)−ab+bc+ca9 Notice that, if a≥b≥c, then cyc∑ba≤cyc∑ab;cyc∑b2a2≤cyc∑a2b2
Therefore, we only need to consider the case a≥b≥c, because the case a≥b≥c after applying (*) will reduce its value N≥ac5+ac2b−ab+bc+ca9>0M+N≥ab8+ac5−ab+bc+ca18>0
Let k=21+5, consider the following cases: (i) The case a−b≤k(b−c). Then we have (a−b)2≤(a−c)(b−c), so (a−b)2M+(c−a)(c−b)N≥(a−b)2(M+N)≥0 (ii) The case a−b≥k(b−c). We only need to prove M≥0 or (a2+b2+6ab)(ab+bc+ca)≥9a2b2
Since (k+1)b−a≤kc and k+1=23+5, we have ab−(a−b)2=[2(3+5)b−a][a−2(3−5)b]≤kc[a+2(3−5)b]≤2c(a+b)