The two pairs are Pythagorean triples because if you plug the two legs of a right triangle into the Pythagorean theorem(A^2+B^2=C^2), then you will find the measurement for the third side(hypotenuse). i.e. 15^2+12^2=9^2(it's the Pythagorean triple 3,4,5 multiplied by 3). This works with any triple, as long as your using the legs of the triangle and as long as the triangle is a right triangle.
