Parallelogram law
Sides and diagonals have equal sums of squares
Parallelogram law
In mathematics, the simplest form of the parallelogram law (also called the parallelogram identity) belongs to elementary geometry. It states that the sum of the squares of the lengths of the four sides of a parallelogram equals the sum of the squares of the lengths of the two diagonals. We use these notations for the sides: AB, BC, CD, DA. But since in Euclidean geometry a parallelogram necessarily has opposite sides equal, that is, AB = CD and BC = DA, the law can be stated as 2 A B 2 + 2 B C 2 = A C 2 + B D 2 {\displaystyle 2AB^{2}+2BC^{2}=AC^{2}+BD^{2}\,} If the parallelogram is a rectangle, the two diagonals are of equal...
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