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CBSE Class 9 Maths Notes Chapter 8 Quadrilaterals PDF Download

Here, we have provided CBSE Class 9 Maths Notes Chapter 8 Quadrilaterals. Students can view these CBSE Class 9 Maths Notes Chapter 8 before exams for better understanding of the chapter.
authorImageAnanya Gupta22 May, 2024
CBSE Class 9 Maths Notes Chapter 8

CBSE Class 9 Maths Notes Chapter 8: In Class 9 Math, Chapter 8 talks about Quadrilaterals, which are shapes with four straight sides. This chapter helps you understand different kinds of quadrilaterals, like parallelograms, rectangles, squares, rhombuses, and trapeziums.

You'll learn about their special features and how to recognize them. We'll also explore things like opposite sides and angles, diagonals, and special properties like symmetry and congruence. By understanding this chapter well, you'll be better prepared for more advanced math topics later on.

CBSE Class 9 Maths Notes Chapter 8 Quadrilaterals Overview

These notes on Chapter 8, Quadrilaterals, are written by subject experts of Physics Wallah in simple language to help students understand the concepts easily. In this chapter, you will learn about different types of quadrilaterals, such as parallelograms, rectangles, squares, rhombuses, and trapeziums. The notes explain their properties, like the relationships between sides and angles, and how to identify each type of quadrilateral. By studying these notes, students can grasp the fundamentals of quadrilaterals, which will be useful for more advanced math topics in the future.

CBSE Class 9 Maths Notes Chapter 8 Quadrilaterals PDF

You can access the CBSE Class 9 Maths Notes for Chapter 8 Quadrilaterals in PDF format using the provided link. These notes provide comprehensive explanations and examples to help you grasp the concepts of triangles effectively.

CBSE Class 9 Maths Notes Chapter 8 Quadrilaterals PDF

CBSE Class 9 Maths Notes Chapter 8 Quadrilaterals

Quadrilaterals

Quadrilaterals are a specific type of polygon characterized by having exactly four sides. These geometric shapes are formed by the union of four line segments. Common examples of quadrilaterals include squares, rectangles, parallelograms, and trapezoids. Each quadrilateral has four vertices (corners) and four angles. The sum of the interior angles of a quadrilateral is always 360 degrees. Understanding the properties and types of quadrilaterals is fundamental in geometry, as they form the basis for more complex shapes and are widely used in various mathematical applications.

Examples of Quadrilaterals

Quadrilaterals

Parallelograms

A parallelogram is a specific type of quadrilateral defined by its unique properties. One defining characteristic of a parallelogram is that its opposite sides are both parallel and equal in length. This means that if you extend the opposite sides indefinitely, they will never intersect. Parallelograms encompass various other quadrilaterals, including rectangles, rhombuses, and squares, each possessing additional properties beyond those of a standard parallelogram. In contrast, a trapezium is another type of quadrilateral, but it differs from a parallelogram in that only one pair of its opposite sides are parallel. Therefore, it does not qualify as a parallelogram. Despite this distinction, trapeziums share similarities with parallelograms, such as having opposite sides of equal length. Understanding the properties and distinctions of parallelograms and trapeziums is crucial in geometry, as they form the basis for many geometric concepts and mathematical calculations.
Parallelogram In the diagram,

Opposite Sides

A B тИе D C ЁЭР┤ЁЭР╡тИеЁЭР╖ЁЭР╢ and A D тИе B C ЁЭР┤ЁЭР╖тИеЁЭР╡ЁЭР╢

A B = D C ЁЭР┤ЁЭР╡=ЁЭР╖ЁЭР╢ and A D = B C ЁЭР┤ЁЭР╖=ЁЭР╡ЁЭР╢

  • Opposite Angles are Equal.

From figure,

A ╦Ж = C ╦Ж ЁЭР┤^=ЁЭР╢^ and B ╦Ж = D ╦Ж ЁЭР╡^=ЁЭР╖^

Diagonals of a Parallelogram Bisect Each Other

Parallelogram Diagonals
In the diagram,

O D = O B ЁЭСВЁЭР╖=ЁЭСВЁЭР╡ and O A = O C ЁЭСВЁЭР┤=ЁЭСВЁЭР╢

Each diagonal divides the parallelogram into two congruent triangles

Congruent Triagles of Parallelogram
In the diagram,

тЦ│ A B C тЙЕ тЦ│ C D A тЦ│ЁЭР┤ЁЭР╡ЁЭР╢тЙЕтЦ│ЁЭР╢ЁЭР╖ЁЭР┤

тЦ│ A B D тЙЕ тЦ│ C D B тЦ│ЁЭР┤ЁЭР╡ЁЭР╖тЙЕтЦ│ЁЭР╢ЁЭР╖ЁЭР╡

Opposite Sides of a Quadrilateral

Opposite Sides of Quadrilateral

Two sides of a quadrilateral, which have no common point, are called opposite sides.

  • In the diagram, A B ЁЭР┤ЁЭР╡ and D C ЁЭР╖ЁЭР╢ is one pair of opposite sides.

  • D C ЁЭР╖ЁЭР╢ and B C ЁЭР╡ЁЭР╢ is the other pair of opposite side

Understanding Quadrilateral Properties

Quadrilaterals are four-sided polygons with distinct properties that define their angles and sides. One fundamental aspect of quadrilaterals is the concept of consecutive sides, opposite angles, and consecutive angles. Consecutive sides are two sides of a quadrilateral that share a common endpoint. For example, in a quadrilateral ABCD, AB and BC form one pair of consecutive sides, while BC and CD, CD and DA, and DA and AB represent the other three pairs of consecutive sides. Opposite angles in a quadrilateral are a pair of angles that do not share a side in their intersection. In the quadrilateral ABCD, angles A and C form one pair of opposite angles, while angles B and D constitute another pair of opposite angles. Consecutive angles, on the other hand, are two angles of a quadrilateral that include a side in their intersection. For instance, angles A and B represent one pair of consecutive angles, while angles B and C, C and D, and D and A form the other three pairs of consecutive angles. Understanding these properties helps in identifying and analyzing quadrilaterals, facilitating geometric calculations and problem-solving in various mathematical contexts.

Theorem 1 Statement

  • The diagonals of a parallelogram bisect each other.

  • If two sides of a triangle are unequal, the longer side has the greater angle opposite to it.

  • A B C D ЁЭР┤ЁЭР╡ЁЭР╢ЁЭР╖ is a parallelogram in which diagonals A C ЁЭР┤ЁЭР╢ and

    B D ЁЭР╡ЁЭР╖

    intersect each other at O ЁЭСВ .

Consecutive angles of Parallelogram

To prove:

The diagonals A C ЁЭР┤ЁЭР╢ and

B D ЁЭР╡ЁЭР╖

bisect eac B ╦Ж ЁЭР╡^ and D ╦Ж ЁЭР╖^ areh other that is,

A O = O C ЁЭР┤ЁЭСВ=ЁЭСВЁЭР╢ and

B O = D O ЁЭР╡ЁЭСВ=ЁЭР╖ЁЭСВ .

Proof:

A B тИе C D ЁЭР┤ЁЭР╡тИеЁЭР╢ЁЭР╖ (By definition of parallelogram)

A C ЁЭР┤ЁЭР╢ is a transversal.

тИ┤ O A ╦Ж B = O C ╦Ж D . . . . . ( i ) тИ┤ЁЭСВЁЭР┤^ЁЭР╡=ЁЭСВЁЭР╢^ЁЭР╖┬а.....┬а(ЁЭСЦ) (Alternate angles are equal in a parallelogram)

Also,

A B = D C ЁЭР┤ЁЭР╡=ЁЭР╖ЁЭР╢ (Opposite sides are equal in a parallelogram)

Now in ╬Ф A O B ╬ФЁЭР┤ЁЭСВЁЭР╡ and $\Delta COD

A B = D C ЁЭР┤ЁЭР╡=ЁЭР╖ЁЭР╢ (Opposite sides of parallelogram are equal)

O A ╦Ж B = O C ╦Ж D ЁЭСВЁЭР┤^ЁЭР╡=ЁЭСВЁЭР╢^ЁЭР╖ (Proved by ( i ) (ЁЭСЦ) )

A O ╦Ж B = C O ╦Ж D ЁЭР┤ЁЭСВ^ЁЭР╡=ЁЭР╢ЁЭСВ^ЁЭР╖ (Vertically opposite angles are equal)

Therefore,

тЦ│ A O B тЙЕ тЦ│ C O D тЦ│ЁЭР┤ЁЭСВЁЭР╡┬атЙЕ┬атЦ│ЁЭР╢ЁЭСВЁЭР╖

( A A S ЁЭР┤ЁЭР┤ЁЭСЖ Congruency condition)

Therefore,

A O = O C ЁЭР┤ЁЭСВ=ЁЭСВЁЭР╢ and B O = O D ЁЭР╡ЁЭСВ=ЁЭСВЁЭР╖ (corresponding parts of congruent triangles are congruent) that is the diagonals of a parallelogram bisect each other.

Sufficient Conditions for a Quadrilateral to be a Parallelogram

Identifying a parallelogram involves understanding its defining properties. These properties provide sufficient conditions for determining whether a quadrilateral is a parallelogram. One key property is that if a quadrilateral is a parallelogram, then its opposite sides are equal. Conversely, if both pairs of opposite sides of a quadrilateral are equal, then the quadrilateral is a parallelogram. Another condition is that if the diagonals of a quadrilateral bisect each other, the quadrilateral is a parallelogram. This means that the point where the diagonals intersect divides each diagonal into two equal parts. Additionally, if either pair of opposite sides of a quadrilateral are equal and parallel, then the quadrilateral is a parallelogram. This condition emphasizes the significance of parallelism in identifying parallelograms. Understanding these conditions enables us to efficiently recognize and classify parallelograms, aiding in geometric analysis and problem-solving.

Theorem 2 2

Statement:

If the diagonals of a quadrilateral bisect each other, then the quadrilateral is a parallelogram.

Consecutive angles of Parallelogram

Given:

A B C D ЁЭР┤ЁЭР╡ЁЭР╢ЁЭР╖ is a quadrilateral in which diagonals A C ЁЭР┤ЁЭР╢ and B D ЁЭР╡ЁЭР╖ intersect at O ЁЭСВ such that A O = O C ЁЭР┤ЁЭСВ=ЁЭСВЁЭР╢ and B O = O D ЁЭР╡ЁЭСВ=ЁЭСВЁЭР╖ .

To prove:

A B C D ЁЭР┤ЁЭР╡ЁЭР╢ЁЭР╖ is a parallelogram.

Proof:

In triangles A O B ЁЭР┤ЁЭСВЁЭР╡ and C O D ЁЭР╢ЁЭСВЁЭР╖ ,

A O = C O ЁЭР┤ЁЭСВ=ЁЭР╢ЁЭСВ (Given)

B O = O D ЁЭР╡ЁЭСВ=ЁЭСВЁЭР╖ (Given)

A O ╦Ж B = C O ╦Ж D ЁЭР┤ЁЭСВ^ЁЭР╡=ЁЭР╢ЁЭСВ^ЁЭР╖ (Vertically opposite angles are equal)

Therefore,

╬Ф A O B тЙЕ ╬Ф C O D ╬ФЁЭР┤ЁЭСВЁЭР╡┬атЙЕ┬а╬ФЁЭР╢ЁЭСВЁЭР╖

( S A S ЁЭСЖЁЭР┤ЁЭСЖ Congruency condition)

Therefore,

O A ╦Ж B = O C ╦Ж D ЁЭСВЁЭР┤^ЁЭР╡=ЁЭСВЁЭР╢^ЁЭР╖ ( c p c t ЁЭСРЁЭСЭЁЭСРЁЭСб )

Since these are alternate angles made by the transversal A C ЁЭР┤ЁЭР╢ intersecting A B ЁЭР┤ЁЭР╡ and C D ЁЭР╢ЁЭР╖

Therefore,

A B тИе C D ЁЭР┤ЁЭР╡тИеЁЭР╢ЁЭР╖

Similarly,

A D тИе B C ЁЭР┤ЁЭР╖тИеЁЭР╡ЁЭР╢

Hence, A B C D ЁЭР┤ЁЭР╡ЁЭР╢ЁЭР╖ is a parallelogram.

Theorem 3 3 :

Statement:

A quadrilateral is a parallelogram if one pair of opposite sides are equal and parallel.

Diagonal of a Parallelogram

Given:

A B C D ЁЭР┤ЁЭР╡ЁЭР╢ЁЭР╖ is a quadrilateral in which A B тИе C D ЁЭР┤ЁЭР╡тИеЁЭР╢ЁЭР╖ and A B = C D ЁЭР┤ЁЭР╡=ЁЭР╢ЁЭР╖ .

To prove:

A B C D ЁЭР┤ЁЭР╡ЁЭР╢ЁЭР╖ is a parallelogram.

Construction:

Join A C ЁЭР┤ЁЭР╢ .

Proof:

In triangles A B C ЁЭР┤ЁЭР╡ЁЭР╢ and A D C ЁЭР┤ЁЭР╖ЁЭР╢ ,

A B = C D ЁЭР┤ЁЭР╡=ЁЭР╢ЁЭР╖ (Given)

B A ╦Ж C = A C ╦Ж D ЁЭР╡ЁЭР┤^ЁЭР╢=ЁЭР┤ЁЭР╢^ЁЭР╖ (Alternate angles are equal)

A C = A C ЁЭР┤ЁЭР╢=ЁЭР┤ЁЭР╢ (Common side)

Therefore,

╬Ф A B C тЙЕ ╬Ф C D A ╬ФЁЭР┤ЁЭР╡ЁЭР╢┬атЙЕ┬а╬ФЁЭР╢ЁЭР╖ЁЭР┤

( S A S ЁЭСЖЁЭР┤ЁЭСЖ Congruency condition)

B C ╦Ж A = D A ╦Ж C ЁЭР╡ЁЭР╢^ЁЭР┤=ЁЭР╖ЁЭР┤^ЁЭР╢ (Corresponding parts of corresponding triangles)

Since these are alternate angles,

A B тИе C D ЁЭР┤ЁЭР╡тИеЁЭР╢ЁЭР╖

Thus, in the quadrilateral A B C D ЁЭР┤ЁЭР╡ЁЭР╢ЁЭР╖ , A B тИе C D ЁЭР┤ЁЭР╡тИеЁЭР╢ЁЭР╖ and A D тИе B C ЁЭР┤ЁЭР╖тИеЁЭР╡ЁЭР╢

Therefore, A B C D ЁЭР┤ЁЭР╡ЁЭР╢ЁЭР╖ is a parallelogram.

Special Parallelograms

Parallelograms encompass a diverse set of quadrilaterals, including rectangles, rhombuses, and squares. Each of these special parallelograms has distinct properties and characteristics.

Rectangle:

A rectangle is a parallelogram with all interior angles measuring 90 degrees, making it a right angle. Consequently, opposite sides of a rectangle are equal in length.

Rhombus:

A rhombus is a parallelogram with all sides of equal length. This means that opposite sides are equal and parallel. However, the angles of a rhombus are not necessarily 90 degrees, except in the case of a square.

Square:

A square is a special case of both a rectangle and a rhombus. It possesses all the properties of a rectangle, including right angles, and all the sides are equal in length like a rhombus.

Relationships Between Special Parallelograms:

In terms of relationships, every rectangle and rhombus is inherently a parallelogram. Therefore, they are depicted as subsets of a parallelogram. Furthermore, because a square possesses the characteristics of both a rectangle and a rhombus, it is represented by the overlapping shaded region in the diagram. Understanding the distinctions and relationships between these special parallelograms is crucial for geometry and problem-solving applications.

Rectangle

A rectangle is a parallelogram with one of its angles as a right angle.

Rectangle

In the above figure,

Let, A ╦Ж = 90 тИШ ЁЭР┤^=90тИШ

Since,

A D тИе B C ЁЭР┤ЁЭР╖тИеЁЭР╡ЁЭР╢ ,

A ╦Ж + B ╦Ж = 180 тИШ ЁЭР┤^+ЁЭР╡^=180тИШ

(Sum of interior angles on the same side of transversal A B ЁЭР┤ЁЭР╡ )

Therefore,

B ╦Ж = 90 тИШ ЁЭР╡^=90тИШ

Here,

A B тИе C D ЁЭР┤ЁЭР╡тИеЁЭР╢ЁЭР╖ and A ╦Ж = 90 тИШ ЁЭР┤^=90тИШ (Given)

Therefore,

A ╦Ж + D ╦Ж = 180 тИШ ЁЭР┤^+ЁЭР╖^=180тИШ

тИ┤ D ╦Ж = 90 тИШ тИ┤ЁЭР╖^=90тИШ

тИ┤ C ╦Ж = 90 тИШ тИ┤ЁЭР╢^=90тИШ

Corollary: Each of the four angles of a rectangle is a right angle.

Rhombus

A rhombus is a parallelogram with a pair of its consecutive sides equal.

Rhombus

A B C D ЁЭР┤ЁЭР╡ЁЭР╢ЁЭР╖ is a rhombus in which A B = B C ЁЭР┤ЁЭР╡=ЁЭР╡ЁЭР╢ .

Since a rhombus is a parallelogram,

A B = D C ЁЭР┤ЁЭР╡=ЁЭР╖ЁЭР╢ and B C = A D ЁЭР╡ЁЭР╢=ЁЭР┤ЁЭР╖

Thus, A B = B C = C D = A D ЁЭР┤ЁЭР╡=ЁЭР╡ЁЭР╢=ЁЭР╢ЁЭР╖=ЁЭР┤ЁЭР╖

Corollary: All the four sides of a rhombus are equal (congruent).

Square

A square is a rectangle with a pair of its consecutive sides equal.

Square

Since square is a rectangle, each angle of a rectangle is a right angle and A B = D C ЁЭР┤ЁЭР╡=ЁЭР╖ЁЭР╢ , B C = C D ЁЭР╡ЁЭР╢=ЁЭР╢ЁЭР╖ .

Thus,

A B = B C = C D = A D ЁЭР┤ЁЭР╡=ЁЭР╡ЁЭР╢=ЁЭР╢ЁЭР╖=ЁЭР┤ЁЭР╖

Each of the four angles of a square is a right angle and each of the four sides is of the same length.

Theorem 4 4

Statement:

The diagonals of a rectangle are equal in length.

Diagonals of a Rectangle

Given:

A B C D ЁЭР┤ЁЭР╡ЁЭР╢ЁЭР╖ is a rectangle.

A C ЁЭР┤ЁЭР╢ and B D ЁЭР╡ЁЭР╖ are diagonals.

To prove:

A C = B D ЁЭР┤ЁЭР╢=ЁЭР╡ЁЭР╖

Proof:

Let, A ╦Ж = 90 тИШ ЁЭР┤^=90тИШ (By definition of rectangle)

A ╦Ж + B ╦Ж = 180 тИШ ЁЭР┤^+ЁЭР╡^=180тИШ (Consecutive interior angle)

A ╦Ж = B ╦Ж = 90 тИШ ЁЭР┤^=ЁЭР╡^=90тИШ

Now in triangles, A B D ЁЭР┤ЁЭР╡ЁЭР╖ and A B C ЁЭР┤ЁЭР╡ЁЭР╢ ,

A B = A B ЁЭР┤ЁЭР╡=ЁЭР┤ЁЭР╡ (Common side)

A ╦Ж = B ╦Ж = 90 тИШ ЁЭР┤^=ЁЭР╡^=90тИШ (Each angle is a right angle)

A D = B C ЁЭР┤ЁЭР╖=ЁЭР╡ЁЭР╢ (Opposite sides of parallelogram)

Therefore,

╬Ф A B D тЙЕ ╬Ф B A C ╬ФЁЭР┤ЁЭР╡ЁЭР╖┬атЙЕ┬а╬ФЁЭР╡ЁЭР┤ЁЭР╢

Therefore,

B D = A C ЁЭР╡ЁЭР╖=ЁЭР┤ЁЭР╢ (Corresponding parts of corresponding triangles)

Hence the theorem is proved.

Converse of Theorem 4 4 :

Statement:

If two diagonals of a parallelogram are equal, it is a rectangle.

Diagonals of Rectangle

Given:

A B C D ЁЭР┤ЁЭР╡ЁЭР╢ЁЭР╖ is a parallelogram in which A C = B D ЁЭР┤ЁЭР╢=ЁЭР╡ЁЭР╖ .

To prove:

Parallelogram A B C D ЁЭР┤ЁЭР╡ЁЭР╢ЁЭР╖ is a rectangle.

Proof:

In triangles A B C ЁЭР┤ЁЭР╡ЁЭР╢ and D B C ЁЭР╖ЁЭР╡ЁЭР╢ ,

A B = D C ЁЭР┤ЁЭР╡=ЁЭР╖ЁЭР╢ (Opposite sides of parallelogram)

B C = B C ЁЭР╡ЁЭР╢=ЁЭР╡ЁЭР╢ (Common side)

A C = B D ЁЭР┤ЁЭР╢=ЁЭР╡ЁЭР╖ (Given)

Therefore,

╬Ф A B C тЙЕ ╬Ф D C B ╬ФЁЭР┤ЁЭР╡ЁЭР╢┬атЙЕ┬а╬ФЁЭР╖ЁЭР╢ЁЭР╡

(

S S S ЁЭСЖЁЭСЖЁЭСЖ

congruency condition)

Therefore,

A B ╦Ж C = D C ╦Ж B ЁЭР┤ЁЭР╡^ЁЭР╢=ЁЭР╖ЁЭР╢^ЁЭР╡ (Corresponding parts of corresponding triangles)

But these angles are consecutive interior angles on the same side of transversal B C ЁЭР╡ЁЭР╢ and A B тИе D C ЁЭР┤ЁЭР╡тИеЁЭР╖ЁЭР╢ .

Therefore,

A B ╦Ж C + D C ╦Ж B = 180 тИШ ЁЭР┤ЁЭР╡^ЁЭР╢+ЁЭР╖ЁЭР╢^ЁЭР╡=180тИШ

But,

A B ╦Ж C = D C ╦Ж B ЁЭР┤ЁЭР╡^ЁЭР╢=ЁЭР╖ЁЭР╢^ЁЭР╡

Therefore,

A B ╦Ж C = D C ╦Ж B = 90 тИШ ЁЭР┤ЁЭР╡^ЁЭР╢=ЁЭР╖ЁЭР╢^ЁЭР╡=90тИШ

Therefore, by definition of rectangle, parallelogram A B C D ЁЭР┤ЁЭР╡ЁЭР╢ЁЭР╖ is a rectangle.

Hence the theorem is proved.

Theorem 5 5 :

Statement:

The diagonals of a rhombus are perpendicular to each other.

Diagonals of Rhombus

Given:

A B C D ЁЭР┤ЁЭР╡ЁЭР╢ЁЭР╖ is a rhombus.

Diagonal A C ЁЭР┤ЁЭР╢ and B D ЁЭР╡ЁЭР╖ intersect at O ЁЭСВ .

To prove:

A C ЁЭР┤ЁЭР╢ and B D ЁЭР╡ЁЭР╖ bisect each other at right angles.

Proof:

A rhombus is a parallelogram such that

AB = DC = AD = BC . . . . . . ( i ) AB = DC = AD = BC┬а......(i)

Also the diagonals of a parallelogram bisect each other.

Hence,

B O = D O ЁЭР╡ЁЭСВ=ЁЭР╖ЁЭСВ and A O = OC . . . . . . ( ii ) ЁЭР┤ЁЭСВ=OC┬а......(ii)

Now, compare triangles A O B ЁЭР┤ЁЭСВЁЭР╡ and A O D ЁЭР┤ЁЭСВЁЭР╖ ,

A B = A D ЁЭР┤ЁЭР╡=ЁЭР┤ЁЭР╖ (From ( i ) (i) above)

B O = D O ЁЭР╡ЁЭСВ=ЁЭР╖ЁЭСВ (From ( ii ) (ii) above)

AO = AO AO = AO (Common side)

Therefore,

╬Ф A O B тЙЕ ╬Ф A O D ╬ФЁЭР┤ЁЭСВЁЭР╡┬атЙЕ┬а╬ФЁЭР┤ЁЭСВЁЭР╖

(

S S S ЁЭСЖЁЭСЖЁЭСЖ

congruency condition)

Therefore,

A O ╦Ж B = A O ╦Ж D ЁЭР┤ЁЭСВ^ЁЭР╡=ЁЭР┤ЁЭСВ^ЁЭР╖

(Corresponding parts of corresponding parts)

B D ЁЭР╡ЁЭР╖

is a straight line segment.

Therefore,

A O ╦Ж B + A O ╦Ж D = 180 тИШ ЁЭР┤ЁЭСВ^ЁЭР╡+ЁЭР┤ЁЭСВ^ЁЭР╖=180тИШ

But,

A O ╦Ж B = A O ╦Ж D ЁЭР┤ЁЭСВ^ЁЭР╡=ЁЭР┤ЁЭСВ^ЁЭР╖

(Proved)

Therefore,

A O ╦Ж B = A O ╦Ж D = 180 тИШ 2 ЁЭР┤ЁЭСВ^ЁЭР╡=ЁЭР┤ЁЭСВ^ЁЭР╖=180тИШ2
A O ╦Ж B = A O ╦Ж D = 90 тИШ ЁЭР┤ЁЭСВ^ЁЭР╡=ЁЭР┤ЁЭСВ^ЁЭР╖=90тИШ

That is, the diagonals bisect at right angles.

Hence the theorem is proved.

Converse of Theorem 5 5 :

Statement:

If the diagonals of a parallelogram are perpendicular then it is a rhombus.

Given:

A B C D ЁЭР┤ЁЭР╡ЁЭР╢ЁЭР╖ is a parallelogram in which A C ЁЭР┤ЁЭР╢ and B D ЁЭР╡ЁЭР╖ are perpendicular to each other.

Diagonals of Rhombus

To prove:

A B C D ЁЭР┤ЁЭР╡ЁЭР╢ЁЭР╖ is a rhombus.

Proof:

Let A C ЁЭР┤ЁЭР╢ and B D ЁЭР╡ЁЭР╖ intersect at right angles at O ЁЭСВ .

A O ╦Ж B = 90 тИШ ЁЭР┤ЁЭСВ^ЁЭР╡=90тИШ

In triangles A O D ЁЭР┤ЁЭСВЁЭР╖ and C O D ЁЭР╢ЁЭСВЁЭР╖ ,

A O = O C ЁЭР┤ЁЭСВ=ЁЭСВЁЭР╢ (Diagonals bisect each other)

O D = O D ЁЭСВЁЭР╖=ЁЭСВЁЭР╖ (Common side)

A O ╦Ж D = C O ╦Ж D = 90 тИШ ЁЭР┤ЁЭСВ^ЁЭР╖=ЁЭР╢ЁЭСВ^ЁЭР╖=90тИШ

(Given)

Therefore,

╬Ф A O D тЙЕ ╬Ф C O D ╬ФЁЭР┤ЁЭСВЁЭР╖┬атЙЕ┬а╬ФЁЭР╢ЁЭСВЁЭР╖

(

S A S ЁЭСЖЁЭР┤ЁЭСЖ

congruency condition)

A D = D C ЁЭР┤ЁЭР╖=ЁЭР╖ЁЭР╢

That is, the adjacent sides are equal.

Therefore, by definition, A B C D ЁЭР┤ЁЭР╡ЁЭР╢ЁЭР╖ is a rhombus.

Hence the theorem is proved.

Benefits of CBSE Class 9 Maths Notes Chapter 8 Quadrilaterals

  • Conceptual Understanding: These notes provide a comprehensive explanation of the properties and characteristics of quadrilaterals, helping students build a strong conceptual foundation in geometry.
  • Clarity in Definitions: By clearly defining terms such as parallelograms, rectangles, rhombuses, and squares, the notes ensure that students understand the distinctions between different types of quadrilaterals.
  • Problem-Solving Skills: Through worked examples and exercises, students can enhance their problem-solving abilities in geometry. Practice questions included in the notes enable students to apply the concepts they've learned to solve a variety of problems.
  • Preparation for Exams: CBSE Class 9 Maths exams often include questions related to quadrilaterals. These notes serve as a valuable resource for exam preparation, ensuring that students are well-equipped to tackle questions on this topic.
CBSE Maths Notes For Class 9
Chapter 1 тАУ Number System
Chapter 2 тАУ Polynomials
Chapter 3 тАУ Coordinate Geometry
Chapter 4 тАУ Linear Equations in Two Variables
Chapter 5 тАУ Introduction to EuclidтАЩs Geometry
Chapter 6 тАУ Lines and Angles
Chapter 7 тАУ Triangles
Chapter 8 тАУ Quadrilaterals
Chapter 9 - Areas of Parallelograms and Triangles
CBSE Class 9 Maths Notes Chapter 10 - Circles Notes
CBSE Class 9 Maths Notes Chapter 11 - Constructions Notes
CBSE Class 9 Maths Notes Chapter 12 - Herons Formula Notes
CBSE Class 9 Maths Notes Chapter 13 - Surface Areas and Volumes Notes
CBSE Class 9 Maths Notes Chapter 14 - Statistics Notes
CBSE Class 9 Maths Notes Chapter 15 - Probability Notes

CBSE Class 9 Maths Notes Chapter 8 FAQs

What are quadrilaterals?

Quadrilaterals are polygons with four sides. They come in various shapes and sizes, such as squares, rectangles, parallelograms, rhombuses, trapezoids, and kites.

What are the properties of a parallelogram?

Parallelograms have opposite sides that are equal and parallel. Their opposite angles are also equal.

What are the conditions for a quadrilateral to be a parallelogram?

A quadrilateral is a parallelogram if its opposite sides are equal and parallel, or if its diagonals bisect each other, or if one pair of opposite sides are equal and parallel.

How can we calculate the area and perimeter of quadrilaterals?

The area of a quadrilateral can be calculated using various formulas depending on the type of quadrilateral. Perimeter is calculated by adding the lengths of all sides. These calculations are essential in fields such as construction, architecture, and engineering.
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