In geometry, Stewart's theorem yields a relation between a lengths of the sides of the triangle and the length of a cevian of the triangle. Its name is in honor of the Scottish mathematician Matthew Stewart who published the theorem in 1746.
Let , , and be the lengths of the sides of a triangle. Let be the length of a cevian to the side of length . If the cevian divides into two segments of length and , with adjacent to and adjacent to , then Stewart's theorem states that
The theorem may be written somewhat more symmetrically using signed lengths of segments, in other words the length AB is taken to be positive or negative according to whether A is to the left or right of B in some fixed orientation of the line. In this formulation, the theorem states that if A, B, and C are collinear points, and P is any point, then
Let θ be the angle between m and d and θ′ the angle between n and d. Then θ′ is the supplement of θ and cos θ′ = −cos θ. The law of cosines for θ and θ′ states
Multiply the first equation by n, the second equation by m, and add to eliminate cos θ, obtaining
which is equation required.
Alternatively, the theorem can proved by drawing a perpendicular from the vertex of the triangle to the base and using the Pythagorean theorem to write the distances b, c, and d in terms of the altitude. The left and right hand sides of the equation then reduce algebraically to the same expression.
- M. Stewart Some General Theorems of Considerable Use in the Higher Parts of Mathematics (1746) "Proposition II"
- Follows Hutton & Gregory or, more closely, PlanetMath.
- This is a overview of the proof in Russell.
5. I.S Amarasinghe, Solutions to the Problem 43.3: Stewart's Theorem(A New Proof for the Stewart's Theorem using Ptolemy's Theorem), Mathematical Spectrum, Vol 43(03), pp.138 - 139, 2011.
6. A. Ostermann, G. Wanner, Further Results in Euclidean Geometry: Problem 14 of Exercises 4.11, Geometry by Its History(Springer books), pp. 112, 2012.
- Hutton, C.; Gregory, O. (1843). A Course of Mathematics II. Longman, Orme & co. p. 219.
- Russell, John Wellesley (1905). "Chapter 1 §3: Stewart's Theorem". Pure Geometry. Clarendon Press. OCLC 5259132.