Inverse Pythagorean theorem

In geometry, the inverse Pythagorean theorem (also known as the reciprocal Pythagorean theorem or the upside down Pythagorean theorem) is as follows: Let A, B be the endpoints of the hypotenuse of a right triangle △ABC. Let D be the foot of a perpendicular dropped from C, the vertex of the right angle, to the hypotenuse. Then 1 C D 2 = 1 A C 2 + 1 B C 2 .

Source: Wikipedia — Inverse Pythagorean theorem (CC BY-SA 4.0)