Menelaus' theorem [SEP] named after
Menelaus's theorem
Menelaus's theorem, named for Menelaus of Alexandria, is a proposition about triangles in plane geometry. Given a triangle "ABC", and a transversal line that crosses "BC", "AC", and "AB" at points "D", "E", and "F" respectively, with "D", "E", and "F" distinct from "A", "B", and "C", then 
or simply
This equation uses
is the only book that has survived, in an Arabic translation. Composed of three books, it deals with the geometry of the sphere and its application in astronomical measurements and calculations. The book introduces the concept of spherical triangle (figures formed of three great circle arcs, which he named "trilaterals") and proves Menelaus' theorem on collinearity of points on the edges of a triangle (which may have been previously known) and its analog for spherical triangles. It was later translated by the sixteenth century astronomer and mathematician