In geometry, Apollonius' theorem is a theorem relating the length of a median of a triangle to the lengths of its side. It states that "the sum of the squares of any two sides of any triangle equals twice the square on half the third side, together with twice the square on the median bisecting the third side".
Specifically, in any triangle ABC, if AD is a median, then
It is a special case of Stewart's theorem. For an isosceles triangle with |AB| = |AC|, the median AD is perpendicular to BC and the theorem reduces to the Pythagorean theorem for triangle ADB (or triangle ADC). From the fact that the diagonals of a parallelogram bisect each other, the theorem is equivalent to the parallelogram law.
The theorem is named for Apollonius of Perga.
Let the triangle have sides a, b, c with a median d drawn to side a. Let m be the length of the segments of a formed by the median, so m is half of a. Let the angles formed between a and d be θ and θ′ where θ includes b and θ′ includes c. Then θ′ is the supplement of θ and cos θ′ = −cos θ. The law of cosines for θ and θ′ states
Add these equations to obtain
- Godfrey, Charles; Siddons, Arthur Warry (1908). Modern Geometry. University Press. p. 20.