# Lines and Angles

## What is a straight line?

All of you have drawn different types of lines while making paintings, playing paper games and doing your homework. But, do you know what exactly a line in terms of Geometry is?

A line is a straight, thin, zero-width object that extends on both sides and has no curves. Here are some examples of straight lines-

### Properties of straight lines

- A straight line travels on a straight path which can be extended indefinitely in both the directions.
- A straight line is shown by two arrowheads in opposite directions.
- A straight line does not have any fixed length.
- A straight line has no beginning point or end point.
- A straight line may have infinite number of points on it.
- A straight line is often denoted by small letters of the English alphabet.

## What is a line segment?

A line segment is a straight path which has a definite length with two endpoints. It is a part of the line.

### 7 Types of straight lines

**Vertical straight lines:**Vertical straight lines go up and down.**Horizontal straight lines:**Horizontal straight lines go from left to right or vice versa.**Perpendicular lines:**Perpendicular lines are straight lines that intersect or cross each other at right angles.**Oblique or slanting lines:**Oblique or slanting lines are straight lines that slant and make a slope.**Transversal lines:**Transversal lines are the lines that intersect two or more other lines.**Intersecting lines:**Intersecting lines are the lines that cross other lines.**Parallel lines:**Parallel lines are the straight lines that never intersect or cross one another.

## What is a ray?

A ray has one end point and infinitely extends in one direction. The Sun’s rays are the perfect examples of rays.

## What is an angle?

An angle is the figure formed by two rays, called the sides of the angle, sharing a common endpoint, called the vertex of the angle.

### 10 Types of angles

**Acute angle:**The angle that is between 0° and 90° is an Acute angle.**Obtuse angle:**The angle that is between 90° and 180° is an Obtuse angle.**Right angle:**The angle that is 90° is a Right angle.**Straight angle:**The angle that is 180° is a Straight angle.**Reflex angle:**The angle that is greater than 180 degrees.**Supplementary angles:**If the sum of two angles is 180° then the angles are called supplementary angles.**Complementary angles:**If the sum of two angles is 90° then the two angles are called complementary angles.**Adjacent angles:**The angles that have a common arm and a common vertex are called adjacent angles.**Linear pair:**The pair of adjacent angles whose sum is a straight angle is called a linear pair.**Vertically opposite angles:**When two lines intersect, the angles formed opposite to each other at the point of intersection (vertex) are called vertically opposite angles.

## What is a curved line?

A curved line is a line that has a curve or arc. Curved lines are not straight lines. Here are some examples of curved lines –

## 7 Interesting facts about lines and angles

- Parallel lines are used in everyday structures that we see around us such as buildings, roads and footpaths.
- The sum of all angles in a Triangle is 180º.
- Adding up all angles in a 4 sided shape such as square/rectangle, parallelogram or quadrilateral is 360º.
- A right angle is denoted by a square mark.
- The word angle is derived from a Latin word ‘angulus’ meaning corner.
- The word ‘acute’ means sharp. The word obtuse means blunt.
- A complete angle or a full angle is known as ‘perigon’. It has 360 degrees or a complete one turn.