**Geometric Figures**

The word "geometry" is derived from two Greek words meaning "earth measure." The basic geometric figures are the point, the line, and the plane. These figures are theoretical and cannot be formally defined. A **point** is represented by a dot and is named by a capital letter. A **line** is an infinite set of points and is named by a small letter or by two points on the line. A **line segment** is part of a line between two points called endpoints. A **plane** is a flat surface (see Fig. 9-1 ).

Points and line segments are used to make geometric figures. The geometric figures presented in this chapter are the triangle, the square, the rectangle, the parallelogram, the trapezoid, and the circle (see Fig. 9-2).

A **triangle** is a geometric figure with three sides. A **rectangle** is a geometric figure with four sides and four 90 ^{°} angles. The opposite sides are equal in length and are parallel. A **square** is a rectangle in which all sides are the same length. A **parallelogram** has four sides with two pairs of parallel sides. A **trapezoid** has four sides, two of which are parallel. A **circle** is a geometric figure such that all the points are the same distance from a point called its center. The center is not part of the circle. A line segment passing through the center of a circle and with its endpoints on the circle is called a **diameter** . A line segment from the center of a circle to the circle is called a **radius** (see Fig. 9-3).

Practice problems for these concepts can be found at: Informal Geometry Practice Test.

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