The cross ratio of four points A, B, C, D is defined by where XY denotes the length of the line segment from X to Y. The idea of a cross ratio goes back at least as far as Pappus of Alexandria (c. 290 – c. 350 AD). Numerous theorems from geometry are stated in terms of the cross ratio. For example, the cross […] The post Cross ratio first appeared on John D. Cook.
