Plane Figures  


Plane Geometry

The part of geometry which studies two-dimensional figures drawn on a flat surface is known as plane geometry. This knowledge is very important because many problems people attempt to solve in everyday life are either two-dimensional by nature, or they can be simplified into two dimensions.

Many problems might appear to be three-dimensional at first glance, but they are actually two-dimensional. One such example is a stream of water coming out of a stationary garden hose or a fire hose. This is because the water is always on a plane - a vertical plane. The elimination of the third dimension makes it much easier to study the projectile motion.

Although many problems are truly three-dimensional, we may simplify them into two-dimensional problems to make a solution easier, if we can do that with reasonable accuracy. For example, plane surveys always treat any small segment of land as a horizontal plane.

When observing nature closely, we find that the seemingly unusual and complicated shapes are merely extensions of a few simple basic figures. In plane geometry, these simple basic shapes include point, line, triangle, quadrilateral, many-sided polygons, and circle. Each has a distinct character of its own. When combined to form a unique object, new properties and useful applications are formed.


A triangle is a closed plane geometric figure formed by connecting the endpoints of three line segments endpoint to endpoint.



A quadrilateral is a four-sided closed plane figure.



A parallelogram is a quadrilateral with both pairs of opposite sides parallel.



A rectangle is a quadrilateral that has four right angles.



When a quadrilateral has four congruent sides, it is called a rhombus. A rhombus is actually an equilateral parallelogram.



A square is a quadrilateral with four congruent sides and four right angles.



If a quadrilateral has only one pair of opposite sides that are parallel, then the quadrilateral is a trapezoid. The parallel sides are called bases. The non-parallel sides are called legs.



Since poly means many and gon means angles, polygon means many angles. In geometry, a polygon is a closed plane figure formed by three or more line segments called sides. Each side intersects exactly two other sides, one at each endpoint. No two sides with a common endpoint are collinear.



A circle is the set of points on a plane that are equidistant from a fixed point known as the center. A circle is named by its center.