# Circle

## Definition

• The locus of points distance $\displaystyle {r}$(radius) from center ${O}$
• A cross section of a cone or cylinder parallel to the base. For more on this, see Dandelin Spheres, a commonly used method of deriving all Conic sections.
• $(x - a)^{2} + (y - b)^{2} = r^{2}$ for a circle centered at $(a,b)$ with radius ${r}$

## Formulas

• Area $\displaystyle \pi r^2$
• circumference $\displaystyle 2\pi r$

## Other Properties

• awaiting diagrams to add stuff on inscribed angles + tangents.