NCERT Book Class 9 Maths Chapter 6 Lines and Angles

CHAPTER 6

LINES AND ANGLES

6.1 Introduction
In Chapter 5, you have studied that a minimum of two points are required to draw a
line. You have also studied some axioms and, with the help of these axioms, you
proved some other statements. In this chapter, you will study the properties of the
angles formed when two lines intersect each other, and also the properties of the
angles formed when a line intersects two or more parallel lines at distinct points.
Further you will use these properties to prove some statements using deductive reasoning
(see Appendix 1). You have already verified these statements through some activities
in the earlier classes.
In your daily life, you see different types of angles formed between the edges of
plane surfaces. For making a similar kind of model using the plane surfaces, you need
to have a thorough knowledge of angles. For instance, suppose you want to make a
model of a hut to keep in the school exhibition using bamboo sticks. Imagine how you
would make it? You would keep some of the sticks parallel to each other, and some
sticks would be kept slanted. Whenever an architect has to draw a plan for a multistoried
building, she has to draw intersecting lines and parallel lines at different angles. Without
the knowledge of the properties of these lines and angles, do you think she can draw
the layout of the building?
In science, you study the properties of light by drawing the ray diagrams.
For example, to study the refraction property of light when it enters from one medium
to the other medium, you use the properties of intersecting lines and parallel lines.
When two or more forces act on a body, you draw the diagram in which forces are
represented by directed line segments to study the net effect of the forces on the
body. At that time, you need to know the relation between the angles when the rays
(or line segments) are parallel to or intersect each other. To find the height of a tower
or to find the distance of a ship from the light house, one needs to know the angle