Geometry: Circles
Khan Academy
Master circles! Learn about circumference, area, theorems, and more with this comprehensive geometry course by Sal Khan.
More resources on Circles
Wolfram MathWorld
MathWorld is an online mathematics encyclopedia from Wolfram Research offering detailed, browsable articles on topics across the math spectrum, including algebra, geometry, calculus, and number theory. Each entry includes definitions, theorems, formulas, diagrams, worked examples, and links to further reading.
mathsisfun.com
MathIsFun.com is a beginner-friendly math site that explains concepts with clear, step-by-step lessons and interactive activities. It covers topics from algebra and linear equations to geometry, fractions, data, and more, with practice problems, worksheets, and quizzes.
Geometry Revisited
Classic Mathematical Association of America text on Euclidean geometry beyond the high-school level, covering triangle centers, cyclic quadrilaterals, collinearity and concurrence, power of a point, transformations and inversion, with exercises that prepare readers for olympiad-style geometry problems.