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### 2.5 Circles

A circle is internally represented by its center and radius.

#### Circle related functions

`circle(A, x)`

  Circle of center A and radius x.

`circle(s)`

  Circle of diameter s.

`radius(c)`

  Radius of circle c.

`perimeter(c)`

  Perimeter of circle c.

`area(c)`

  Area enclosed in circle c.

`arg(A, c)`

  Polar angle of point A with respect to the center of c (in degrees).

`point(c, a)`

  Point on c with polar angle a with respect to its center.

`center(c)`

  Center of circle c.

`line(c, a)`

  Tangent line to circle c. The contact point has polar angle a with respect to the center of c.