Formulas in Plane Geometry Select a derivation below. Log in to post comments Derivation / Proof of Ptolemy's Theorem for Cyclic Quadrilateral Derivation of Formula for Area of Cyclic Quadrilateral Derivation of Formula for Radius of Circumcircle Derivation of Formula for Radius of Incircle Derivation of Heron's / Hero's Formula for Area of Triangle Book traversal links for Formulas in Plane Geometry Derivation of Formula for the Future Amount of Ordinary Annuity Up Derivation / Proof of Ptolemy's Theorem for Cyclic Quadrilateral