Heron's Formula and Ptolemy's Theorem

Abstract

Summary. The goal of this article is to formalize some theorems that are in the Freek Wiedijk’s list ”Formalizing 100 Theorems” on the web. These are elementary theorems included in every handbook of Euclidean geometry and trigonometry: the law of cosines, the Heron’s formula, the isosceles triangle theorem, the intersecting chords theorem and the Ptolemy’s theorem.