Consider an equation for which we seek only integer solutions. There is no standard technique of solving such a problem, though there are some common heuristics that you may apply. A simple example is . Suppose we wish to find out the integer solutions to this equation.

First notice that if ‘x’ and ‘y’ are solutions, so are ‘-x’ and ‘-y’ (and vice versa). So it is sufficient to investigate positive solutions.

The factor method relies on the following steps:

- First bring all variables to one side of the equality sign and constants to the other side.
- Try to factorise left hand expression and compare it with the factorisation of the right hand constant

**Illustration**

But 31 is a prime. So the only way 31 can be written as a product of two positive number is 1 times 31.

Since x-y is smaller, the only possibility is x-y=1, x+y=31, giving solutions x=16, y=15

**Problems**

- ; find all integer x, y that satisfies the solutions

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