Understanding Quadrilaterals Class 8 Notes- Chapter 3

CBSE Class 8 Maths Chapter 3 Understanding Quadrilaterals Notes:-Download PDF Here

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Introduction to Class 8 Understanding Quadrilaterals

In class 8, the chapter “Understanding Quadrilaterals”, will discuss the fundamental concepts related to quadrilaterals, different types of quadrilaterals and their properties, different types of curves, polygons and some of the theorems related to quadrilaterals such as angle sum property of quadrilaterals, and so on, with complete explanation.

What are Quadrilaterals?

Quadrilaterals are one type of polygon which has four sides and four vertices and four angles along with 2 diagonals. There are various types of quadrilaterals.

Quadrilaterals

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Types of Quadrilaterals

The classification of quadrilaterals are dependent on the nature of sides or angles of a quadrilateral and they are as follows:

  • Trapezium
  • Kite
  • Parallelogram
  • Square
  • Rectangle
  • Rhombus

Types of Quadrilaterals

The figure given below represents the properties of different quadrilaterals.

Properties of quadrilateral

For More Information on Types of Quadrilaterals, Watch The Below Video.

 

Revisiting Geometry

As we know, Geometry is one of the branches of Mathematics, that deals with the study of different types of shapes, their properties, and how to construct lines, angles and different polygons. Geometry is broadly classified into plane geometry(two-dimensional) and solid geometry (three-dimensional geometry).

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Introduction to Curves

A curve is a geometrical figure obtained when a number of points are joined without lifting the pencil from the paper and without retracing any portion. It is basically a line which need not be straight.

The various types of curves are:

  • Open curve: An open curve is a curve in which there is no path from any of its point to the same point.
  • Closed curve: A closed curve is a curve that forms a path from any of its point to the same point.

A curve can be :

  • A closed curve:

Closed Curve

  •  An open curve:

Open curves

  • Simple open and closed curves:

Simple closed and open curves

 
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Polygons

A simple closed curve made up of only line segments is called a polygon.
Various examples of polygons are Squares, Rectangles, Pentagons etc.
Note:
The sides of a polygon do not cross each other.

Classification of Polygons on the Basis of Number of Sides / Vertices

Polygons are classified according to the number of sides they have. The following lists the different types of polygons based on the number of sides they have:

  • When there are three sides, it is triangle
  • When there are four sides, it is quadrilateral
  • When there are fives sides, it is pentagon
  • When there are six sides, it is hexagon
  • When there are seven sides, it is heptagon
  • When there are  eight sides, it is octagon
  • When there are nine sides, it is nonagon
  • When there are ten sides, it is decagon

Different Polygons

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Diagonals

A diagonal is a line segment connecting two non-consecutive vertices of a polygon.

Diagonals of Different Polygons

Polygons on the Basis of Shape

Polygons can be classified as concave or convex based on their shape.

  • A concave polygon is a polygon in which at least one of its interior angles is greater than 90. Polygons that are concave have at least some portions of their diagonals in their exterior.
  • A convex polygon is a polygon with all its interior angle less than 180. Polygons that are convex have no portions of their diagonals in their exterior.

Convex and concave polygon

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Polygons on the Basis of Regularity

Polygons can also be classified as regular polygons and irregular polygons on the basis of regularity.

  • When a polygon is both equilateral and equiangular it is called as a regular polygon. In a regular polygon, all the sides and all the angles are equal. Example: Square
  • A polygon which is not regular i.e. it is not equilateral and equiangular, is an irregular polygon. Example: Rectangle

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Regular and Irregular polygon

Angle Sum Property of a Polygon

According to the angle sum property of a polygon, the sum of all the interior angles of a polygon is equal to (n2)×180, where is the number of sides of the polygon.

Angle sum property - 1

As we can see for the above quadrilateral, if we join one of the diagonals of the quadrilateral, we get two triangles.
The sum of all the interior angles of the two triangles is equal to the sum of all the interior angles of the quadrilateral, which is equal to 360 = (42)×180.
So, if there is a polygon which has sides, we can make (n – 2) non-overlapping triangles which will perfectly cover that polygon.

Angle sum property - 2

The sum of the interior angles of the polygon will be equal to the sum of the interior angles of the triangles = (n2)×180

To know More About Angle Sum Property of Triangle, Watch the Below Video

 

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Sum of Measures of Exterior Angles of a Polygon

The sum of the measures of the external angles of any polygon is 360.

Properties of Parallelograms

Parallelogram

The following are the important properties of parallelogram:

  1. The opposite sides of a parallelogram are equal and congruent.
  2. Diagonals of a parallelogram bisect each other.
  3. The diagonals of parallelogram bisect each other and produce two congruent triangles
  4. The opposite angles of a parallelogram are congruent.

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For More Information On Properties Of Parallelogram, Watch The Below Video.


Elements of a Parallelogram

Elements of parallelogram

  • There are four sides and four angles in a parallelogram.
  • The opposite sides and opposite angles of a parallelogram are equal.
  • In the parallelogram ABCD, the sides ¯¯¯¯¯¯¯¯AB and ¯¯¯¯¯¯¯¯¯CD are opposite sides and the sides ¯¯¯¯¯¯¯¯AB and ¯¯¯¯¯¯¯¯BC are adjacent sides.
  • Similarly, ABC and ADC are opposite angles and  ABC and BCD are adjacent angles.

 

Angles of a Parallelogram

The opposite angles of a parallelogram are equal.
In the parallelogram ABCD, ABC=ADC and DAB=BCD.

The adjacent angles in a parallelogram are supplementary.
In the parallelogram ABCD, ABC+BCD=ADC+DAB=180

Angles of a parallelogram

For example, 

Angles of parallelogram - Example

In the given parallelogram (RING), ∠R = 70°. Now, we have to find the remaining angles.

As we know, the opposite angles of a parallelogram are equal, we can write:

 ∠R = ∠N = 70°.

And we know, the adjacent angles of a parallelogram are supplementary, we get

 ∠R + ∠I = 180°

Hence, ∠I = 180° – 70° = 110°

Therefore, ∠I = ∠G = 110° [Since ∠I and ∠G are opposite angles]

Hence the angles of a parallelogram are ∠R = ∠N = 70° and ∠I = ∠G = 110°.

Diagonals of a Parallelogram

The diagonals of a parallelogram bisect each other at the point of intersection.
In the parallelogram ABCD given below, OA = OC and OB = OD.

Diagonals of Parallelogram

Consider an example, if OE = 4cm and HL is five more than PE, find the measure of OH.

Diagonals of a parallelogram - Example

Given that, OE = 4 cm and hence, OP = 4cm [Since OE = OP]

Hence PE =OE + OP = 4cm + 4cm = 8 cm

Also given that, HL is 5 more than PE, 

Hence, HL = 5 + 8 = 13 cm.

Therefore, OH = HL/2 = 13/2 = 6.5 cm

Therefore, the measurement of OH is 6.5 cm

Properties of Special Parallelograms

Rectangle

A rectangle is a parallelogram with equal angles and each angle is equal to 90.
Properties:

  • Opposite sides of a rectangle are parallel and equal.
  • The length of diagonals of a rectangle is equal.
  • All the interior angles of a rectangle are equal to 90.
  • The diagonals of a rectangle bisect each other at the point of intersection.

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Rectangle

For More Information On Rectangle, Watch The Below Video.


Square

A square is a rectangle with equal sides. All the properties of a rectangle are also true for a square.
In a square the diagonals:

  • bisect one another
  • are of equal length
  • are perpendicular to one another

To learn more about squares, click here.

Square

Rhombus

Rhombus is one of the special cases of parallelogram. In Rhombus, all the sides are equal and the opposite sides are also equal.

Rhombus

To learn more about rhombus, click here.

Frequently asked Questions on CBSE Class 8 Maths Notes Chapter 3: Understanding Quadrilaterals

What is a ‘Curve’?

A curve refers to a line that is not straight. In other words, it may be any line that is bent to some extent.

What is a ‘convex polygon’?

Convex polygon is a polygon each of whose angles is less than a straight angle.

What are the properties of a ‘Parallelogram’?

1. Opposite sides are congruent 2. Opposite angels are congruent 3. Consecutive angles are supplementary. 4. Daigonals of a parallelogram bisect

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