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Algebra Difficulty 5.5 AIME, harder Prove it Vietnam

Let a,b,c2a, b, c \ge 2 be real numbers, satisfying the condition a+b+c=1a+1b+1c+8a + b + c = \frac{1}{a} + \frac{1}{b} + \frac{1}{c} + 8. Prove that we have the inequality
3(ab+bc+ca)81(a+b+c)2. 3(ab + bc + ca) \le 81 \le (a + b + c)^2.

Solution

Putting x=a+b+cx = a + b + c then we have a+b+c=1a+1b+1c+89a+b+c+8x9x+8a + b + c = \frac{1}{a} + \frac{1}{b} + \frac{1}{c} + 8 \ge \frac{9}{a + b + c} + 8 \Rightarrow x \ge \frac{9}{x} + 8.
Direct manipulation leads to (x9)(x+1)0(x - 9)(x + 1) \ge 0 or x9x \ge 9.
Moreover, a+b+c=1a+1b+1c+8<12+12+12+8=192a + b + c = \frac{1}{a} + \frac{1}{b} + \frac{1}{c} + 8 < \frac{1}{2} + \frac{1}{2} + \frac{1}{2} + 8 = \frac{19}{2}.
Thus 9x<1929 \le x < \frac{19}{2}. Now, we rewrite the given condition as:
2x=2a+2b+2c+16a2a+b2b+c2c=192x. 2x = \frac{2}{a} + \frac{2}{b} + \frac{2}{c} + 16 \Leftrightarrow \frac{a - 2}{a} + \frac{b - 2}{b} + \frac{c - 2}{c} = 19 - 2x.
By Cauchy-Schwarz inequality, we have
a2a+b2b+c2c[(a2)+(b2)+(c2)]2a(a2)+b(b2)+c(c2)=(x6)2a2+b2+c22x. \frac{a - 2}{a} + \frac{b - 2}{b} + \frac{c - 2}{c} \ge \frac{[(a - 2) + (b - 2) + (c - 2)]^2}{a(a - 2) + b(b - 2) + c(c - 2)} = \frac{(x - 6)^2}{a^2 + b^2 + c^2 - 2x}.
Thus, using previous equation, we have
192x(x6)2a2+b2+c22xa2+b2+c22x+(x6)2192x. 19 - 2x \ge \frac{(x - 6)^2}{a^2 + b^2 + c^2 - 2x} \Rightarrow a^2 + b^2 + c^2 \ge 2x + \frac{(x - 6)^2}{19 - 2x}.
In the other hands, the required inequality is equivalent to
542(ab+bc+ca)54+(a2+b2+c2)x2. 54 \ge 2(ab + bc + ca) \Leftrightarrow 54 + (a^2 + b^2 + c^2) \ge x^2.
By previously proved inequality, it is sufficient to prove that
54+2x+(x6)2192xx2. 54 + 2x + \frac{(x - 6)^2}{19 - 2x} \ge x^2.
Simple calculation leads us to the equivalent inequality
(x9)(x22x59)0, (x - 9)(x^2 - 2x - 59) \ge 0,
which is true due x22x59=(x1)2608260>0x^2 - 2x - 59 = (x - 1)^2 - 60 \ge 8^2 - 60 > 0.
The equality holds in both sides if and only if a=b=c=3a = b = c = 3.

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