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Algebra Difficulty 4.9 AIME Prove it Ukraine

For nonnegative real numbers a,b,ca, b, c, with the sum that does not exceed 22, prove
ab(a2+b2)+bc(b2+c2)+ca(c2+a2)2. ab(a^2 + b^2) + bc(b^2 + c^2) + ca(c^2 + a^2) \leq 2.

Solution

It is enough to prove the inequality for a+b+c=2a + b + c = 2.
Let us denote x=ab+bc+acx = ab + bc + ac, then
a2+b2+c2=(a+b+c)22(ab+bc+ac)=42(ab+bc+ac)=2(2x). a^2 + b^2 + c^2 = (a + b + c)^2 - 2(ab + bc + ac) = 4 - 2(ab + bc + ac) = 2(2 - x).
From the obvious inequality x(2x)1x(2 - x) \leq 1, we get
2+2abc22x(2x)=(ab+bc+ac)(a2+b2+c2)==ab(a2+b2)+cb(c2+b2)+ac(a2+c2)+abc(a+b+c), \begin{aligned} 2 + 2abc &\geq 2 \geq 2x(2 - x) = (ab + bc + ac)(a^2 + b^2 + c^2) = \\ &= ab(a^2 + b^2) + cb(c^2 + b^2) + ac(a^2 + c^2) + abc(a + b + c), \end{aligned}
Using the equality 2abc=abc(a+b+c)2abc = abc(a + b + c), we arrive at the conclusion.

Equality occurs, for example, a=b=1a = b = 1, c=0c = 0.

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