What could be the length of the third side of a triangle that has two sides of lengths 5 and 13?

To create a triangle, the sum of the two shorter sides must be greater than the third side.

If the side of length 13 is the longest side then the missing side must be greater than 13 - 5 = 8

If the missing side is the longest side then the missing side must be less than 13 + 5 = 18

Thus any length that is greater than 8 and less than 18

Examples include: 9, 12, 17

If the third side is 12, the triangle is a Pythagorean triangle.