# A gem of an inequality from USSR !!!

Problem:

The positive numbers a, b, c, A, B, C satisfy

$a+A=b+B=c+C=k$.

Prove that $aB+bC+cA \leq k^{2}$.

All Soviet Union Olympiad, 1987.

Solution:

Proving that $aB+bC+cA \leq k^{2}$ is equivalent to showing that

$aB+bC+cA = a(k-b)+b(k-c)+c(k-a) \leq k^{2}$

$\Longleftrightarrow k(a+b+c) \leq k^{2}+ab+bc+ca$

But this is equivalent to

$k^{2}-ka+bc \geq (b+c)(k-a) \Longleftrightarrow bc \geq (k-a)(b+c-k)$

If $b+c-k \leq 0$, then the above is obviously true. If not, then we have

$(k-b)(k-c) \geq 0 \Rightarrow bc \geq k(b+c-k) \geq (k-a)(b+c-k)$

and thus, our inequality holds.

More later,

Nalin Pithwa

## 4 thoughts on “A gem of an inequality from USSR !!!”

1. Neat. I’m seeing an alternate solution where you start with the identity describing the inequality of arithmetic and geometric means $\sqrt{xy} \leq (x + y)/2$ in the equivalent form

$xy \leq (x + y)^2 / 4$

which applied to $aA, bB, cC$ yields

$aA \leq (a + A)^2/4 = k^2 / 4$
$bB \leq (b + B)^2/4 = k^2 / 4$
$cC \leq (c + C)^2/4 = k^2 / 4$

and then summing these we get the slightly stronger inequality

$aA + bB + cC \leq 3k^2 / 4$

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• Thanks for participating !!

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2. Oh, and there, 3s later I see I misread the identity…Nevermind.

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• Yes, many thanks.

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