A nine-sided polygon. The sum of the angles of a nonagon is 1260 degrees.
Examples:
A regular nonagon:
| An irregular nonagon:
|
Decagon
A ten-sided polygon. The sum of the angles of a decagon is 1440 degrees.
Examples:
A regular decagon:
| An irregular decagon:
|
Circle
A circle is the collection of points in a plane that are all the same distance from a fixed point. The fixed point is called the center. A line segment joining the center to any point on the circle is called a radius.
Example:
The blue line is the radius r, and the collection of red points is the circle.
Convex
A figure is convex if every line segment drawn between any two points inside the figure lies entirely inside the figure. A figure that is not convex is called a concave figure.
Example:
The following figures are convex.
The following figures are concave. Note the red line segment drawn between two points inside the figure that also passes outside of the figure.
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