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.