NCERT Class 8 Mathematics Mathematics: Chapter 3 — Understanding Quadrilaterals
This chapter introduces students to the fundamental concepts of quadrilaterals and polygons. It begins by defining a plane curve and a polygon as a simple closed curve made of line segments. The chapter distinguishes between convex and concave polygons, explaining that convex polygons have all diagonals entirely within the polygon, while concave polygons do not. It also defines regular polygons as those that are both equiangular and equilateral, contrasting them with irregular polygons. Examples like squares, rectangles, and equilateral triangles are used to illustrate these definitions. The chapter aims to build a strong foundation for understanding geometric shapes, crucial for CBSE Mathematics learning.
Quick info
| Board | CBSE / NCERT |
|---|---|
| Class | Class 8 |
| Subject | Mathematics |
| Book | Mathematics |
| Chapter | Chapter 3 — Understanding Quadrilaterals |
| Language | English |
| PDF type | NCERT Textbook |
| Session | CBSE 2026 |
| Reading time | 2 minutes |
| Word count | 361 |
Learning outcomes
- Define and identify polygons, simple curves, and simple closed curves.
- Differentiate between convex and concave polygons.
- Define and identify regular and irregular polygons.
- Understand the properties of squares, rectangles, and rhombuses in relation to regular polygons.
Vocabulary
| Word | Meaning |
|---|---|
| Polygon | A simple closed curve made up of only line segments. |
| Convex polygon | A polygon where all diagonals lie entirely within the polygon. |
| Concave polygon | A polygon where at least one diagonal lies partially or entirely outside the polygon. |
| Regular polygon | A polygon that is both equiangular (all angles equal) and equilateral (all sides equal). |
| Equiangular | Having all angles equal. |
| Equilateral | Having all sides of equal length. |
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Practice questions
- What is a polygon? Answer: A polygon is a simple closed curve made up of only line segments.
- Give an example of a regular polygon. Answer: A square is a regular polygon.
- What is the difference between a convex and a concave polygon? Answer: In a convex polygon, all diagonals lie within the polygon. In a concave polygon, at least one diagonal lies outside.
- What two conditions must a polygon satisfy to be called regular? Answer: A polygon must be both equiangular and equilateral to be regular.
Frequently asked questions
What is a simple closed curve in the context of polygons?
A simple closed curve is one that does not intersect itself and ends at the starting point. A polygon is a simple closed curve made only of line segments.
How can we identify a convex polygon?
A convex polygon is identified by the property that all its diagonals lie completely inside the polygon.
What makes a polygon 'regular'?
A polygon is regular if it has all sides equal in length (equilateral) and all interior angles equal in measure (equiangular).
Is a rectangle a regular polygon? Why or why not?
No, a rectangle is not a regular polygon because although it is equiangular (all angles are 90 degrees), it is not necessarily equilateral (all sides may not be equal).
What is an example of an equilateral but not equiangular polygon?
A rhombus is an example of a quadrilateral that is equilateral but not necessarily equiangular.
What are the basic shapes discussed in this chapter?
The chapter discusses basic shapes like polygons, quadrilaterals, triangles, squares, rectangles, and rhombuses.
Related resources
Important topics
Topics covered
NCERT Class 8 Mathematics — Mathematics — Chapter 3 — Understanding Quadrilaterals. Verified by NCERT Help Editorial Team. Reviewed on 29 Jul 2026. Last updated 10 Aug 2026.