Terms Related to Lines and Angles, Types of Lines and Angles

# Terms Related to Lines and Angles, Types of Lines and Angles

## Terms Related to Lines and Angles

### Point

A dot made with a sharp pencil represents a point. A point has no length or breadth. It shows a position. Points are represented with capital letters such as A, B, C, X, Y, Z, P, Q, etc.

### Line

A line is an infinite number of points joined together. It may extend indefinitely in any direction. It is denoted by the symbol ↔. Lines are named with small letters such as l, m, n, etc.

## Types of Lines

### Straight Line and Curved Line

Take a piece of thread. Stretch the thread tightly from the two opposite ends. You will get a straight line. Now let it loose, you will get a curved line.

### Vertical, Horizontal and Oblique Lines

The standing lines, sleeping lines and slanting lines are called vertical lines, horizontal lines and oblique lines respectively.

### Parallel Lines

The lines m and n have the same distance between them. If they are extended indefinitely in both directions and they do not meet, then they are called parallel lines. We represent parallel lines m and n as m n.

### Intersecting Lines

The lines m and n intersect or cross at O. They are called intersecting lines.

### Perpendicular Lines

When two lines meet or intersect each other at right angles, we say that they are perpendicular. For example, building or a pole is perpendicular to the ground. We represent perpendicular lines l and m as l m.

### Ray

A line which has one end point and extends endlessly in other direction is called a ray. Its end point is called initial point. A ray is represented by the symbol →.

### Line Segment

The straight path between two points is called a line segment. It is the shortest path between two points.

## What is an Angle?

Take a point O. Draw two rays OA and OB. The point O is called the vertex, and OA and OB are called the arms. AOB forms an angle. The space between the arms is called the interior of the angle, whereas the space outside is called the exterior of the angle. The arms of an angle are shown with arrow heads.

### Naming an Angle

When naming an angle the symbol for angle ‘’ is written before the name. The angle is named with the vertex in the middle of the name.
For example, in the following figure, the angle is named as XYZ or ZYX, where Y is the vertex, and YX and YZ are the arms.

### Interior and Exterior of an Angle

Consider any AOB. It divides the region of the plane into three parts.

1. The shaded portion of the plane which is within the two arms of AOB is called the
interior of AOB. As the two rays OA and OB extend endlessly, the interior region also extends endlessly.
2. The portion of the plane which is outside the two arms of AOB is called the exterior of AOB. The exterior region also extends endlessly.
3. The part of the plane consisting of all points lying on the arms OA and OB is called the
boundary of AOB.
4. The interior of AOB along with its boundary is called the angular region.

### Unit of an Angle

An angle is measured in degree (°). A degree is subdivided into 60 equal parts and each part is called a minute (). A minute can be divided into 60 equal parts called seconds ().
Remember that, 1° = 60and 1= 60.

## Types of Angles

### Acute angle

An angle whose measure is greater than 0° but less than 90°, is called an acute angle.

### Right angle

An angle whose measure is exactly 90°, is called a right angle.

### Obtuse angle

An angle whose measure is more than 90° but less than 180°, is called an obtuse angle.

### Straight angle

An angle whose measure is exactly 180°, is called a straight angle.

### Reflex angle

An angle whose measure is more than 180° but less than 360°, is called a reflex angle.

### Complete angle

An angle whose measure is 360°, is called a complete angle.