Lines and Angles Introduction

Geometry is composed of two ancient Greek words: Geo and Metron. Geo means Earth and Metron means measurement. Geometry is the branch of mathematics which deals with shape, size, position, spatial relationships and properties of different figures.

The entire geometry begins with a point. A point is a dimensionless entity which specifies the location or position. It is represented using a dot symbol and its length is zero. All the shapes that we see around us consist of infinite number of points. When a point moves in such a manner that its direction remains unaltered then a straight line is obtained. In other words, a one-dimensional collection of points extending infinitely in both the directions represents a line as shown below. A line never ends.

Any two points on a line can uniquely specify it. In the fig. 1 given above, a line passing through these two points A and B is denoted as \(\overleftrightarrow{AB}\). The arrows indicate that the line \(\overleftrightarrow{AB}\)  is extending infinitely in both the directions.

A portion of a line consisting of two end points is known as a line segment. Fig. 1 given above  represents a line segment \(\overline{AB}\) with A and B as two end points.

Ray is defined as a line segment with only one end point. Fig. 2 represents a ray with O as its end point and one side extending infinitely.

If a ray is rotated about its end point then the measure of its rotation between the final and initial position of the ray is known as an angle. In fig. 3, \(\overrightarrow{OB}\)  is the initial position of the ray and when it is  rotated about its end point i.e. O, the final position is represented by ray \(\overrightarrow{OA}\). The measure of this rotation is measured in angles. The angle between the initial and final position of a ray is measured as ∠AOB.