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The **intersecting chords theorem** or just the **chord theorem** is a statement in elementary geometry that describes a relation of the four line segments created by two intersecting chords within a circle.
It states that the products of the lengths of the line segments on each chord are equal.
It is Proposition 35 of Book 3 of Euclid's *Elements*.

More precisely, for two chords *AC* and *BD* intersecting in a point *S* the following equation holds:

The converse is true as well, that is if for two line segments *AC* and *BD* intersecting in S the equation above holds true, then their four endpoints *A*, *B*, *C* and *D* lie on a common circle. Or in other words if the diagonals of a quadrilateral *ABCD* intersect in *S* and fulfill the equation above then it is a cyclic quadrilateral.

The value of the two products in the chord theorem depends only on the distance of the intersection point *S* from the circle's center and is called the absolute value of the power of *S*, more precisely it can be stated that:

where *r* is the radius of the circle, and *d* is the distance between the center of the circle and the intersection point *S*. This property follows directly from applying the chord theorem to a third chord going through *S* and the circle's center *M* (see drawing).

The theorem can be proven using similar triangles (via the inscribed-angle theorem). Consider the angles of the triangles *ASD* and *BSC*:

This means the triangles *ASD* and *BSC* are similar and therefore

Next to the tangent-secant theorem and the intersecting secants theorem the intersecting chord theorem represents one of the three basic cases of a more general theorem about two intersecting lines and a circle - the power of point theorem.

- Paul Glaister:
*Intersecting Chords Theorem: 30 Years on*. Mathematics in School, Vol. 36, No. 1 (Jan., 2007), p. 22 (JSTOR) - Bruce Shawyer:
*Explorations in Geometry*. World scientific, 2010, ISBN 9789813100947, p. 14 - Hans Schupp:
*Elementargeometrie.*Schöningh, Paderborn 1977, ISBN 3-506-99189-2, p. 149 (German). *Schülerduden - Mathematik I*. Bibliographisches Institut & F.A. Brockhaus, 8. Auflage, Mannheim 2008, ISBN 978-3-411-04208-1, pp. 415-417 (German)

*Intersecting Chords Theorem*at cut-the-knot.org*Intersecting Chords Theorem*at proofwiki.org- Weisstein, Eric W. "Chord".
*MathWorld*. - Two interactive illustrations: [1] and [2]