This document explains how to determine if a parallelogram is a rectangle and calculates the distance between two points using the Pythagorean theorem and cross product.
Extract
[...] Consider the triangle POT and determine if this triangle is a right triangle at O. We therefore , and therefore, according to the Pythagorean theorem, the triangle POT is right-angled at O. The parallelogram STOP therefore has a right angle, and since a parallelogram with at least one right angle is a rectangle, STOP is a rectangle. We need to calculate BC to find the distance from B to C. Consider the rectangle ABC which is right-angled at B. According to the Pythagorean theorem, we have: CBA The distance from point B to point C is approximately 233 meters. [...]
