Geometry Formulas and Theorems for Circles

By Mark Ryan

Part of Geometry Workbook For Dummies Cheat Sheet

Geometry isn’t all about pointy angles — there are circles, too. What’s interesting about circles isn’t just their roundness: Become familiar with geometry formulas that help you measure angles around circles, as well as their area and circumference. Following are the formulas you need to know about circles:


And, circles have their own theorems as well:

  • Chord-Chord Power Theorem: When two chords intersect, the products of the measures of their parts are equal.

  • Tangent-Secant Power Theorem: When a tangent and a secant meet at an external point, the measure of the tangent squared is equal to the product of the secant’s external part and its total length.

  • Secant-Secant Power Theorem: When two secants meet at an external point, the products of their external parts and their total lengths are equal.