Circle
Equation of a circle
x² + y² = r², shown here as x² + y² = 9
A circle is the set of all points a fixed distance r (the radius) from a center point. When the center sits at the origin, the equation reduces to x² + y² = r²; for a center at (h, k) it's (x − h)² + (y − k)² = r². This example uses r = 3, so r² = 9.
Also written parametrically as x = r cos(t), y = r sin(t) for t from 0 to 2π.