Overview
Plane figures in geometry include points, segments, lines, rays, and angles, polygons, curves, and circles. They appear on a flat plane.
Points, Segments, Lines, Rays, and Angles
Think of a pencil point, and make it smaller, until it is a no-dimensional, undefined, geometric concept that only exists in the minds of mathematicians. Segments are made up of points, and they consist of the beginning, the end and all points between the beginning and the end. They exist in two dimensions. Extend the segment to infinity in both directions, and it is a line. Start from only one point in any direction and extend it to infinity, and it is a ray. Rays are usually designated by the beginning point and the endpoint of a line segment. Two rays that meet at a common point, the vertex, are called an angle, and the angle can be measured with a protractor or duplicated using a drawing compass.
Polygons
A three-sided polygon is called a triangle, because it consists of line segments arranged in three angles. Four-sided polygons may be squares, if all four sides are equal and each side meets at right angles. Other types of four-sided polygons include the rectangle, the rhombus, the parallelogram, and the trapezoid. Polygons are usually named for the number of sides they have; such as a pentagon with 5 sides, a hexagon with 6 sides, a heptagon with 7 sides, and octagon with 8 sides, and so on.
Curves and Circles
A set of points connected by a continuous line is called a curve. Curves can be open or closed. A circle is a type of closed curve that has a single center. Each point along the circumference of the circle is at the same distance from the center. The distance from the center to the circle itself is called the radius, and any circle measures 360o.
Other Curved Figures
Many other curved figures exist in geometry and in the real world, such as ellipses, arches, ovals, lenses, and crescents. Technically, polygons are also curves, because they can be formed by a continuous line, even if that line doesn’t appear curved.
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