Menelaus's theorem
In Euclidean geometry, Menelaus's theorem, named for Menelaus of Alexandria, is a proposition about triangles in plane geometry. Suppose we have a triangle , and a transversal line that crosses  at points  respectively, with  distinct from . A weak version of the theorem states that

where "| |" denotes absolute value (i.e., all segment lengths are positive). The theorem can be strengthened to a statement about signed lengths of segments, which provides some additional information about the relative order of collinear points. Here, the length  is taken to be positive or negative according to whether  is to the left or right of  in some fixed orientation of the line; for example,  is defined as having positive value when  is between  and  and negative otherwise. The signed version of Menelaus's theorem states

Equivalently,

Some authors organize the factors differently and obtain the seemingly different relation

but as each of these factors is the negative of the corresponding factor above, the relation is seen to be the same. The converse is also true: If points  are chosen on  respectively so that 

then  are collinear. The converse is often included as part of the theorem. (Note that the converse of the weaker, unsigned statement is not necessarily true.) The theorem is very similar to Ceva's theorem in that their equations differ only in sign. By re-writing each in terms of cross-ratios, the two theorems may be seen as projective duals. Proofs

A standard proof 
First, the sign of the left-hand side will be negative since either all three of the ratios are negative, the case where the line  misses the triangle (lower diagram), or one is negative and the other two are positive, the case where  crosses two sides of the triangle. (See Pasch's axiom.) To check the magnitude, construct perpendiculars from  to the line  and let their lengths be  respectively. Then by similar triangles it follows that 

Therefore,

For a simpler, if less symmetrical way to check the magnitude, draw  parallel to  where  meets  at . Then by similar triangles

and the result follows by eliminating  from these equations. The converse follows as a corollary. Let  be given on the lines  so that the equation holds. Let  be the point where  crosses . Then by the theorem, the equation also holds for . Comparing the two, 

But at most one point can cut a segment in a given ratio so

A proof using homothecies 
The following proof uses only notions of affine geometry, notably homothecies. Whether or not  are collinear, there are three homothecies with centers  that respectively send  to ,  to , and  to . The composition of the three then is an element of the group of homothecy-translations that fixes , so it is a homothecy with center , possibly with ratio 1 (in which case it is the identity). This composition fixes the line  if and only if  is collinear with  (since the first two homothecies certainly fix , and the third does so only if  lies on ). Therefore  are collinear if and only if this composition is the identity, which means that the magnitude of product of the three ratios is 1:

which is equivalent to the given equation. History
It is uncertain who actually discovered the theorem; however, the oldest extant exposition appears in Spherics by Menelaus. In this book, the plane version of the theorem is used as a lemma to prove a spherical version of the theorem. In Almagest, Ptolemy applies the theorem on a number of problems in spherical astronomy. During the Islamic Golden Age, Muslim scholars devoted a number of works that engaged in the study of Menelaus's theorem, which they referred to as "the proposition on the secants" (shakl al-qatta). The complete quadrilateral was called the "figure of secants" in their terminology. Al-Biruni's work, The Keys of Astronomy, lists a number of those works, which can be classified into studies as part of commentaries on Ptolemy's Almagest as in the works of al-Nayrizi and al-Khazin where each demonstrated particular cases of Menelaus's theorem that led to the sine rule, or works composed as independent treatises such as:

 The "Treatise on the Figure of Secants" (Risala fi shakl al-qatta''') by Thabit ibn Qurra. Husam al-Din al-Salar's Removing the Veil from the Mysteries of the Figure of Secants (Kashf al-qina' 'an asrar al-shakl al-qatta'), also known as "The Book on the Figure of Secants" (Kitab al-shakl al-qatta) or in Europe as The Treatise on the Complete Quadrilateral. The lost treatise was referred to by Sharaf al-Din al-Tusi and Nasir al-Din al-Tusi. Work by al-Sijzi. Tahdhib'' by Abu Nasr ibn Iraq. Roshdi Rashed and Athanase Papadopoulos, Menelaus'  Spherics:  Early Translation and al-Mahani'/al-Harawi's version (Critical edition of Menelaus' Spherics from the Arabic manuscripts, with historical and mathematical commentaries), De Gruyter, Series: Scientia Graeco-Arabica,  21,  2017, 890 pages. References

External links 

 Alternate proof of Menelaus's theorem, from PlanetMath
 Menelaus From Ceva
 Ceva and Menelaus Meet on the Roads
 Menelaus and Ceva at MathPages
 Demo of Menelaus's theorem by Jay Warendorff. The Wolfram Demonstrations Project. Affine geometry
Theorems about triangles
Articles containing proofs
Euclidean plane geometry