Geometry Formulas and Theorems for Circles
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:

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

TangentSecant 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.

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