Maths Olympiad Prep

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Number theory Difficulty 5.9 AIME, harder Prove it Romania

Find all the sequences of equal ratios of the form a1a2=a3a4=a5a6=a7a8\frac{a_1}{a_2} = \frac{a_3}{a_4} = \frac{a_5}{a_6} = \frac{a_7}{a_8} fulfilling the conditions:
- the set {a1,a2,,a8}\{a_1, a_2, \dots, a_8\} is the set of the positive divisors of 2424;
- the common value of the ratios is an integer.

Solution

The common value rr of the ratios can be only a divisor of 2424, different from 11; these divisors are 22, 33, 44, 66, 88, 1212 and 2424.
If r=2r = 2, then 2=2412=84=63=212 = \frac{24}{12} = \frac{8}{4} = \frac{6}{3} = \frac{2}{1};
if r=3r = 3, then 3=248=124=62=313 = \frac{24}{8} = \frac{12}{4} = \frac{6}{2} = \frac{3}{1};
if r=4r = 4, then 4=246=123=82=414 = \frac{24}{6} = \frac{12}{3} = \frac{8}{2} = \frac{4}{1}.
There are no other sequences for r=2r = 2, r=3r = 3 or r=4r = 4, because if we order decreasingly the divisors and use them one by one, we have to put at the numerator the largest divisor dd still unused, and at the denominator dr\frac{d}{r}, getting the sequences from above.
There is no sequence for r8r \ge 8, respectively r=6r = 6, because none of the equalities d6=r\frac{d}{6} = r and 6d=r\frac{6}{d} = r, respectively d3=6\frac{d}{3} = 6 and 3d=6\frac{3}{d} = 6, can be fulfilled.

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