Maths Olympiad Prep

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Algebra Difficulty 5.3 AIME, harder Prove it United States

Problem:

Find all ordered triples (a,b,c)(a, b, c) of positive reals that satisfy: abc=3\lfloor a\rfloor b c=3, abc=4a\lfloor b\rfloor c=4, and abc=5a b\lfloor c\rfloor=5, where x\lfloor x\rfloor denotes the greatest integer less than or equal to xx.

Solution

Solution:

Answer: (303,304,2305),(303,302,305)\left(\frac{\sqrt{30}}{3}, \frac{\sqrt{30}}{4}, \frac{2 \sqrt{30}}{5}\right),\left(\frac{\sqrt{30}}{3}, \frac{\sqrt{30}}{2}, \frac{\sqrt{30}}{5}\right)

Write p=abcp=a b c, q=abcq=\lfloor a\rfloor\lfloor b\rfloor\lfloor c\rfloor. Note that qq is an integer.

Multiplying the three equations gives:
p=60q p=\sqrt{\frac{60}{q}}
Substitution into the first equation,
p=3aa<3a+1a6 p=3 \frac{a}{\lfloor a\rfloor}<3 \frac{\lfloor a\rfloor+1}{\lfloor a\rfloor} \leq 6
Looking at the last equation:
p=5cc5cc5 p=5 \frac{c}{\lfloor c\rfloor} \geq 5 \frac{\lfloor c\rfloor}{\lfloor c\rfloor} \geq 5
Here we've used xx<x+1\lfloor x\rfloor \leq x<\lfloor x\rfloor+1, and also the apparent fact that a1\lfloor a\rfloor \geq 1. Now:
560q6125q53 \begin{aligned} & 5 \leq \sqrt{\frac{60}{q}} \leq 6 \\ & \frac{12}{5} \geq q \geq \frac{5}{3} \end{aligned}
Since qq is an integer, we must have q=2q=2. Since qq is a product of 3 positive integers, we must have those be 1, 1, and 2 in some order, so there are three cases:

Case 1: a=2\lfloor a\rfloor=2. By the equations, we'd need a=2330=120/9>3a=\frac{2}{3} \sqrt{30}=\sqrt{120 / 9}>3, a contradiction, so there are no solutions in this case.

Case 2: b=2\lfloor b\rfloor=2. We have the solution
(303,302,305) \left(\frac{\sqrt{30}}{3}, \frac{\sqrt{30}}{2}, \frac{\sqrt{30}}{5}\right)

Case 3: c=2\lfloor c\rfloor=2. We have the solution
(303,304,2305) \left(\frac{\sqrt{30}}{3}, \frac{\sqrt{30}}{4}, \frac{2 \sqrt{30}}{5}\right)

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