## Quick Reference

A closed curve every point on which is a fixed distance (the **radius**) from a point (the **centre**) within the curve. The **diameter** is a line that joins two points on the **circumference** and passes through the centre: the diameter is twice the radius (*r*). The circumference of a circle is equal to 2π*r*; the area is π*r*^{2}, where π is a constant with the value 3.141 592. In analytical geometry the equation of a circle, centred at the origin, is *x*^{2} + *y*^{2} = *r*^{2}.

**A circle.**

**From:**
circle
in
A Dictionary of Physics »

*Subjects:*
Mathematics — Physics.

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