The Pythagorean Theorem states that in a right triangle, the square of the length of the hypotenuse (the side opposite the right angle) is equal to the sum of the squares of the lengths of the other two sides. Mathematically, this is expressed as:
c^2 = a^2 + b^2
Where c is the length of the hypotenuse, and a and b are the lengths of the triangle's other two sides.
The converse of the Pythagorean Theorem is the reverse statement:
If the square of the length of one side of a triangle is equal to the sum of the squares of the lengths of the other two sides, then the triangle is a right triangle.
In other words, if you have a triangle where c^2 = a^2 + b^2, then that triangle must have a right angle, and it's a right triangle.