Find all positive integers and such that
are both integers.
Solution
By the symmetry of the problem, we may suppose that . Notice that , so that if is a positive integer, then . Rearranging this inequality and factorizing, we find that . Since , we must have .
We therefore have two cases:
Case 1: . Substituting, we have
which is an integer if and only if . As , the only possible values are or . Hence, or .
Case 2: . Substituting, we have
Once again, notice that , and hence, for to be an integer, we must have , that is, . Hence, since is an integer, we can bound by . Checking all the ordered pairs , we find that only and satisfy the given conditions.
Thus, the ordered pairs that work are
where the last two pairs follow by symmetry.
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