Euler's Quadrilateral Theorem

Euler's quadrilateral theorem, also called Euler's law on quadrilaterals, is named after Leonhard Euler (1707-1783) and describes a relation between the sides of a convex quadrilateral and its two diagonals. It generalizes the parallelogram law, which is itself a generalization of the Pythagorean theorem, and for that reason the underlying result is sometimes called the Euler-Pythagoras theorem. This description is adapted from Wikipedia contributors under CC BY-SA 4.0; changes were made. https://creativecommons.org/licenses/by-sa/4.0/