A positive integer only contains the digits , and . Prove that this integer is not a perfect square.
Solution
The last digit of a perfect square can be , , , , or . Denote the given number by . If is to be a perfect square it has to end in .
Assume that the last digits of are equal to and the digit just to the left of them is not . If is odd, . Then is also a perfect square. It is divisible by so it must also be divisible by . This is not possible since the second digit from the right is either or .
Hence, must end in an even number of zeros and is even. In this case is a positive integer and a perfect square with the last digit equal to either or . Again, this is not possible. Hence, is never a perfect square.
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