Example 6 Find all positive integers , such that the system of equations
has integer solutions.
Example 6 Find all positive integers , such that the system of equations
has integer solutions.
Solving by rearranging and completing the square, the system of equations transforms to
Since 50 can be expressed as the sum of squares of two positive integers in only two ways: , it follows from (1) that or 7, and from (2) that or 7, thus or 13.
Furthermore, for each , we have or 13. Depending on , we consider three cases.
If , then from (1) we know , and from (2) we know , and so on. Thus, when , ; when , ; and when , . Therefore, by the second equation, we know that the original system of equations has integer solutions if and only if , i.e., if and only if , meets the requirement.
For the other two cases and , similar discussions yield the same condition.
In summary, the that satisfies the condition are all multiples of 3.