What is a two-dimensional figure?
A two-dimensional figure, also called a plane or planar figure, is a set of line segments or sides and curve segments or arcs, all lying in a single plane. The sides and arcs are called the edges of the figure. The edges are one-dimensional, but they lie in the plane, which is two-dimensional. The endpoints of the edges are called the vertices or corners. These points are zero-dimensional, but they also lie in the plane, which is two-dimensional. The most common figures have only a few edges, the curves are very simple, and there are no “loose ends” – that is, every vertex is the endpoint of at least two edges. If all the edges are segments, every vertex is the endpoint of two sides, and no two sides cross each other, the figure is called a polygon.1 Polygons are classified according to the number of sides they have, which equals the number of vertices. Here are some names of polygons. Polygons often divide the plane into two pieces, an inside and an outside. The inside part is called
