Free application to calculate any diagonal or side value of all the squares and rectangles and the value of the sides of a right triangle with pythagorean theorem You just have to field two side values and click on the CALCUL button, the last side value will be calculated. In mathematics, the Pythagorean theorem, also known as Pythagoras' theorem, is a fundamental relation in Euclidean geometry among the three sides of a right triangle. It states that the square of the hypotenuse (the side opposite the right angle) is equal to the sum of the squares of the other two sides. A perfect application if you are a student or a scientist.