NCERT Book Class 8 Maths Chapter 3 Understanding Quadrilaterals

UNDERSTANDING QUADRILATERALS 21
CHAPTER
Understanding
Quadrilaterals
3
3.1 Introduction
You know that the paper is a model for a plane surface. When you join a number of
points without lifting a pencil from the paper (and without retracing any portion of the
drawing other than single points), you get a plane curve.

3.1.1 Convex and concave polygons
A simple closed curve made up of only line segments is called a polygon.

Curves that are polygons Curves that are not polygons
Here are some convex polygons and some concave polygons. (Fig 3.1)

Convex polygons Concave polygons
Fig 3.1
Can you find how these types of polygons differ from one another? Polygons that are
convex have no portions of their diagonals in their exteriors or any line segment joining any
two different points, in the interior of the polygon, lies wholly in the interior of it . Is this
true with concave polygons? Study the figures given. Then try to describe in your own
words what we mean by a convex polygon and what we mean by a concave polygon. Give
two rough sketches of each kind.
In our work in this class, we will be dealing with convex polygons only.
3.1.2 Regular and irregular polygons
A regular polygon is both ‘equiangular’ and ‘equilateral’. For example, a square has sides
of equal length and angles of equal measure. Hence it is a regular polygon. A rectangle is