# 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. There are different parameter involved to define a shape. The three important parameters are lines and angles. In this article, we are going to discuss two important parameters called lines and angles in detail.

## Definition of Lines and Angles

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.

### What are Lines?

All the shapes that we see around us consist of an 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 endpoints is known as a line segment. Fig. 1 given above represents a line segment $\overline{AB}$ with A and B as two endpoints.

Ray:

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

### What is an Angle?

If a ray is rotated about its endpoint 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 endpoint 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.

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