Maths Olympiad Prep

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Algebra Difficulty 5.3 AIME, harder Prove it New Zealand

Problem:
Let aa, bb and cc be positive real numbers such that a+b+c=abca + b + c = abc. Prove that at least one of aa, bb or cc is greater than 1710\frac{17}{10}.

Solution

Solution:
Wlog assume abca \geq b \geq c. Therefore a+b+c3ca + b + c \geq 3c. Now for a proof by contradiction, assume a1710a \leq \frac{17}{10} and b1710b \leq \frac{17}{10}. Since (1710)2=289100<3\left(\frac{17}{10}\right)^2 = \frac{289}{100} < 3, it follows that 1710<3\frac{17}{10} < \sqrt{3} and therefore ab3ab \leq 3. However this implies:

3ca+b+c=abc<3c3c \leq a + b + c = abc < 3c

which is a contradiction.

1=a+b+cabc=1bc+1ca+1ab1 = \frac{a + b + c}{abc} = \frac{1}{bc} + \frac{1}{ca} + \frac{1}{ab}
As all three terms are positive, at least one must be less than or equal to 1/31/3 and, without loss of generality, we can assume 1/bc1/31/bc \leq 1/3. But then bc3bc \geq 3 and at least one of bb or cc must be greater than 3\sqrt{3}. Since 3>17/10\sqrt{3} > 17/10 we're done.

By the AM-GM inequality we have a+b+c3abc3\frac{a + b + c}{3} \geq \sqrt[3]{abc} and thus abc3abc3abc \geq 3\sqrt[3]{abc}, which rearranges to give abc33\sqrt[3]{abc} \geq \sqrt{3}. Now wlog abca \geq b \geq c and so

a=a33abc33>1710.a = \sqrt[3]{a^3} \geq \sqrt[3]{abc} \geq \sqrt{3} > \frac{17}{10}.

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Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.