# Lines and Angles Notes - Class 9

## Class 9: Lines and Angles Notes - Class 9

The document Lines and Angles Notes - Class 9 is a part of Class 9 category.
All you need of Class 9 at this link: Class 9

What is lines and angles?
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Lines

Basic Terms and Definitions

• A line segment is a part of a line with two end-points. It is the shortest distance between two points and has a fixed length. • A ray is a part of a line, which has a starting point and extends infinitely in one direction. • Collinear points are a set of three or more points which lie on the same line.
• The points which do not lie on the same line are called non-collinear points.

It is to be noted that, three points can be either collinear or non-collinear, but not both together at a same time.

Angles

Basic Terms and Definitions

• An angle is a figure in which two rays emerge from a common point. This point is called the vertex of the angle and the two rays forming the angle are called its arms or sides.
• If the arms form an angle of 90 degrees between them, it is called a right angle.
• If the inclination between the arms is less than a right angle, it is called an acute angle.
• If the inclination between the arms is more than a right angle, it is called an obtuse angle.
• If the arms form an angle of 180 degrees between them, it is called a straight angle.
• An angle which is greater than 180 degrees but less than 360 degrees is called a reflex angle.
• Two angles which sum up to 90 degrees are called complementary angles.
• Two angles which sum up to 180 degrees are called supplementary angles.
• Two angles which have a common side and a common vertex are called adjacent angles. In the following figure, ∠α and ∠β are adjacent angles. • If two adjacent angles add up to 180 degrees, they form a linear pair of angles. In the following figure, ∠a and ∠b form a linear pair. • When two lines intersect each other, the two opposite pairs of angles formed are called vertically opposite angles. In the following figure, ∠A and ∠B are vertically opposite angles. Another pair is ∠C and ∠D. The document Lines and Angles Notes - Class 9 is a part of Class 9 category.
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