Maths Olympiad Prep

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Number theory Difficulty 5.1 AIME, harder Prove it United States

Problem:

Determine, with proof, whether or not there exist positive integers aa, bb, and cc such that
ab+bc=acandabc=10!. a b + b c = a c \quad \text{and} \quad a b c = 10! .

Solution

Solution:

The answer is no. Note that 10!10! has exactly one prime factor of 77. Therefore, exactly one of aa, bb, and cc is divisible by 77. If 77 divides bb (we write this as 7∣b7 \mid b), then 7∣ab+bc7 \mid a b + b c but 7∤ac7 \nmid a c, so the equation cannot hold. Likewise, if 7∣a7 \mid a, then 7∣ab7 \mid a b, 7∣ac7 \mid a c but 7∤bc7 \nmid b c, and if 7∣c7 \mid c, then 7∣bc7 \mid b c, 7∣ac7 \mid a c but 7∤ab7 \nmid a b. So there are no solutions.

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