If this is a right triangle, then

where c is the hypotenuse which is ALWAYS THE LONGEST SIDE. Our longest side is 5, so we will set up Pythagorean's Theorem to see if the addition of the squares of a and b = the square of c:

. Squaring all of that and adding on the left gives us 25=25. Because this is true, we have proven the converse of the Pythagorean Theorem here and this is a right triangle.