Maths Olympiad Prep

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Number theory Difficulty 5.2 AIME, harder Prove it Slovenia

A positive integer only contains the digits 00, 33 and 77. Prove that this integer is not a perfect square.

Solution

The last digit of a perfect square can be 00, 11, 44, 55, 66 or 99. Denote the given number by nn. If nn is to be a perfect square it has to end in 00.

Assume that the last kk digits of nn are equal to 00 and the digit just to the left of them is not 00. If kk is odd, k=2m1k = 2m - 1. Then n102m2\frac{n}{10^{2m-2}} is also a perfect square. It is divisible by 1010 so it must also be divisible by 2525. This is not possible since the second digit from the right is either 33 or 77.

Hence, nn must end in an even number of zeros and kk is even. In this case n10k\frac{n}{10^k} is a positive integer and a perfect square with the last digit equal to either 33 or 77. Again, this is not possible. Hence, nn is never a perfect square.

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