For how many ordered triples of positive integers are the equations and satisfied?
Solution
Subtracting the first equation from the second, we obtain . Since , and are positive integers, at least one must equal 1. Note that is not a valid triple, so it suffices to consider the cases where exactly two or one of are equal to 1. If , we obtain and similarly for the other two cases, so this gives 3 ordered triples. If , then we need , which has 6 solutions for ; a similar argument for and gives a total of 18 such solutions. It is easy to check that all the solutions we found are actually solutions to the original equations. Adding, we find total triples.
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