Abstract
This entry provides a geometric proof of the intersecting chords
theorem. The theorem states that when two chords intersect each other
inside a circle, the products of their segments are equal. After a
short review of existing proofs in the literature, I decided to use a
proof approach that employs reasoning about lengths of line segments,
the orthogonality of two lines and the Pythagoras Law. Hence, one can
understand the formalized proof easily with the knowledge of a few
general geometric facts that are commonly taught in high-school. This
theorem is the 55th theorem of the Top 100 Theorems list.