Exercise 13 - 11.13 | Chapter 3 : Quadrilaterals | CBSE Class 9 Maths
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Angle Sum Property of Quadrilaterals

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Teacher
Teacher

Today, we're going to discuss the angle sum property of quadrilaterals. Can anyone tell me what the sum of the interior angles of any quadrilateral is?

Student 1
Student 1

Is it 360 degrees?

Teacher
Teacher

Correct! The sum is always 360 degrees. Let's say we have a quadrilateral with three angles measuring 90 degrees, 85 degrees, and 95 degrees. What would the fourth angle be?

Student 2
Student 2

360 minus the sum of those angles, so 360 - (90 + 85 + 95) = 90 degrees!

Teacher
Teacher

Excellent! Remember, the formula you can use is: Fourth Angle = 360 - (Angle1 + Angle2 + Angle3).

Properties of Parallelograms

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Teacher
Teacher

Now, let's recall the properties of parallelograms. What can you tell me about the opposite sides?

Student 3
Student 3

Opposite sides are equal and parallel.

Teacher
Teacher

Very good! And what about the angles?

Student 4
Student 4

Opposite angles are equal.

Teacher
Teacher

That's right! So, if one angle is 70 degrees, what are the others?

Student 1
Student 1

The opposite angle would also be 70 degrees, and the adjacent angles would be 110 degrees each, right?

Teacher
Teacher

Exactly! Great work connecting that!

Diagonal Properties in Rectangles

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Teacher
Teacher

Let's now discuss rectangles. Can anyone tell me what makes rectangles special regarding their diagonals?

Student 2
Student 2

The diagonals are equal and bisect each other.

Teacher
Teacher

That's correct! If we have a rectangle, and one diagonal measures 10 cm, what does the other diagonal measure?

Student 3
Student 3

It would also be 10 cm!

Teacher
Teacher

Exactly! They are equal in length. Remember, to prove that diagonals bisect each other, you can use congruence in triangles formed.

Applying the Mid-point Theorem

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Teacher
Teacher

Who can remind us what the midpoint theorem states?

Student 4
Student 4

It states that the line segment joining the midpoints of two sides of a triangle is parallel to the third side and half its length.

Teacher
Teacher

Correct! If we have a triangle ABC, and D and E are midpoints of sides AB and AC, what can we conclude about DE?

Student 1
Student 1

DE would be parallel to BC and half its length!

Teacher
Teacher

Exactly! This theorem helps us solve many problems involving triangles and quadrilaterals. Great job!

Introduction & Overview

Read a summary of the section's main ideas. Choose from Basic, Medium, or Detailed.

Quick Overview

This section addresses various exercises related to quadrilaterals, reinforcing concepts learned in Chapter 11 about their properties and calculations.

Standard

Exercise 13 covers multiple problems that enhance understanding of quadrilaterals, particularly focusing on calculating angles, properties of specific types of quadrilaterals, and applying theorems like the midpoint theorem. The exercises facilitate practical applications of the properties and theorems discussed in earlier parts of the chapter.

Detailed

Detailed Summary

In this section, we explore a variety of exercises that focus on the properties and applications of quadrilaterals. Quadrilaterals are polygons with four sides, and understanding their properties is essential for solving geometry problems. This exercise section includes:

  1. Angle Calculation: Problems that require students to calculate the missing angles in quadrilaterals using the angle sum property, which states that the sum of the interior angles of a quadrilateral is always 360Β°.
  2. Parallelogram Properties: Exercises involve finding missing angles and demonstrating the properties of parallelograms, such as the equality of opposite sides and angles.
  3. Diagonals of Quadrilaterals: Problems that reinforce the concept that the diagonals of a rectangle are equal and bisect each other.
  4. Midpoint Theorem: One exercise focuses on applying the midpoint theorem, which states that the line segment joining the mid-points of two sides of a triangle is parallel to the third side and half its length.
  5. Area Calculation: Some tasks require students to calculate the area of various quadrilaterals (like parallelograms, rhombuses, trapeziums, and kites) using the appropriate formulas.

Overall, these exercises not only test theoretical knowledge but also familiarize students with necessary geometric calculations.

Audio Book

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Understanding the Problem

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A square and a rectangle have equal areas. If the side of the square is 14 cm, find the breadth of the rectangle if its length is 28 cm.

Detailed Explanation

To find the breadth of the rectangle, first calculate the area of the square. The area of a square is given by the formula: Area = side Γ— side. In this case, the side of the square is 14 cm, so the area = 14 cm Γ— 14 cm = 196 cmΒ².

Since the square and rectangle have equal areas, the area of the rectangle is also 196 cmΒ². The area of a rectangle is calculated using the formula: Area = length Γ— breadth. Here, the length of the rectangle is given as 28 cm. Thus, we can set up the equation: 196 cmΒ² = 28 cm Γ— breadth. To find the breadth, we simply rearrange the equation to: breadth = Area Γ· length = 196 cmΒ² Γ· 28 cm = 7 cm.

Examples & Analogies

Imagine you have a square garden that each side measures 14 cm. You want to build a rectangular garden that covers the same area as your square garden. Knowing one side (the length) of your rectangular garden is 28 cm, you need to figure out how wide (the breadth) it should be to ensure it takes up the same space as your square garden.

Using the Area Formulas

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Area of a square = side Γ— side; Area of a rectangle = length Γ— breadth.

Detailed Explanation

The area of any shape gives you a measure of how much space is inside it. For a square, since all sides are equal, you multiply the length of one side by itself. For the rectangle, you multiply its length by its breadth to find the area. In this exercise, you first used the area formula for the square to find a specific value, then applied the rectangle's area formula to relate back to the square's area.

Examples & Analogies

Think of the area of a square like painting a tabletop. If you know how wide and long it is (which are the same for a square), you can easily figure out how much paint you'll need. Now, if it's a rectangular surface, it’s like knowing how long and wide a banner is to cover the same space; you might need to do a little extra math if the lengths differ.

Final Calculation

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Hence, the breadth of the rectangle is 7 cm.

Detailed Explanation

By performing the necessary calculations, we arrive at the final answer for the breadth of the rectangle. It's crucial to perform the division correctly to get to the result of 7 cm. This step shows how we can manipulate formulas to work with known measurements and find unknown values.

Examples & Analogies

Think of needing a specific amount of soil to fill a rectangular flower bed. Knowing the dimensions helps to determine exactly how much soil to use. Just like in this problem, finding one missing dimension from known areas can be likened to solving simple garden layout challenges where certain dimensions must match to accomplish the same task.

Definitions & Key Concepts

Learn essential terms and foundational ideas that form the basis of the topic.

Key Concepts

  • Angle Sum Property: The sum of all interior angles of a quadrilateral equals 360 degrees.

  • Properties of Parallelograms: Opposite sides and angles are equal, and diagonals bisect each other.

  • Rectangle Characteristics: Diagonals are equal in length and bisect one another.

  • Mid-point Theorem: The segment connecting the midpoints of a triangle's two sides is parallel to the third side and half its length.

Examples & Real-Life Applications

See how the concepts apply in real-world scenarios to understand their practical implications.

Examples

  • Example 1: Calculate the missing angle in a quadrilateral with angles 110Β°, 85Β°, and 95Β°: Fourth angle = 360 - (110 + 85 + 95) = 70Β°.

  • Example 2: In a parallelogram with one angle measuring 60Β°, the remaining angles would be: 60Β°, 120Β°, 60Β°, 120Β°.

Memory Aids

Use mnemonics, acronyms, or visual cues to help remember key information more easily.

🎡 Rhymes Time

  • For a quadrilateral, sum of angles is nice, it’s always 360, won't that suffice?

πŸ“– Fascinating Stories

  • A clever quadrilateral named Quaddy found that despite its shape, it could always find that the total of its angles was 360 degrees, living happily in geometry's land.

🧠 Other Memory Gems

  • Remember: SPARK for rectangles - Sides equal, Parallel, Angles 90Β°, Rectangle is a special kind!

🎯 Super Acronyms

PARA for parallelograms - Pairs of opposite sides equal, Angles opposite are equal, Diagonals bisect, A parallelogram it is!

Flash Cards

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Glossary of Terms

Review the Definitions for terms.

  • Term: Quadrilateral

    Definition:

    A polygon that has four sides, four vertices, and four angles.

  • Term: Parallelogram

    Definition:

    A quadrilateral with opposite sides that are both parallel and equal in length.

  • Term: Rectangle

    Definition:

    A type of parallelogram in which all angles are right angles.

  • Term: Square

    Definition:

    A quadrilateral with all sides equal and all angles equal to 90 degrees.

  • Term: Angle Sum Property

    Definition:

    The property stating that the sum of the interior angles of a quadrilateral is 360 degrees.

  • Term: Diagonals

    Definition:

    Line segments that connect non-adjacent vertices in a polygon.

  • Term: Midpoint Theorem

    Definition:

    The theorem stating that the segment joining two midpoints of a triangle is parallel to the third side and half its length.