# CBSE Class 8 Maths Revision Notes Chapter 13

## CBSE Class 8 Mathematics Revision Notes Chapter 13 – Direct and Inverse Proportions

CBSE Class 8 Mathematics Chapter 13 notes– Direct and Inverse Proportion will assist the students of Class 8 to understand the basic concepts of direct and inverse proportion.  In addition, in this Class 8 chapter 13 Mathematics notes, students will get to know the significant details of the chapter that are important for their final examination. Along with Chapter 13 Mathematics Class 8 notes, Extramarks will provide students with essential questions that can be asked to prepare them quickly. Moreover, Class 8 Mathematics notes Chapter 13 will be a student’s last-minute revision guide providing all the necessary information. These notes are based on the CBSE syllabus

## Revision Notes for CBSE Class 8 Mathematics Chapter 13 – Free PDF Download

### Access Class 8 Mathematics Chapter 13–Direct and Inverse Proportions

When the values of two quantities depend on one another, such that the change in one quantity causes a change in the other, the two quantities are said to be in variation.

Direct Proportion:

• Two quantities named a and b will be in direct proportion if they vary, i.e., they increase or decrease together so that the ratio of their corresponding values remains constant.
• If ab=c  [c is a positive number), then a and b are said to change directly.
• To explain the condition, let a1,a2be the two values of a and b1,b2 be the values of b, then

a1 b1=a2b2

• We write a ∝ b to represent two quantities that are related proportionately,

Some examples of Direct Proportion:

• The total distance covered by a car will increase if the quantity of petrol in a car increases.
• The area of that circle will increase if the radius of a circle increases.

Inverse Proportions:

• The two quantities a and b are said to be in inverse proportion if an increment in one causes a proportional decrement in the other or if a decrement in one causes a proportional increment in the other; that is, the product of their corresponding values remains constant. This means that if ab=c or a=cb (c is a constant), then a and b are said to vary inversely.
• To explain this case, let, be the two values of a and, b1,b2 be the values of b, then a1,b1=,a2 b2
• We write a∝1b to represent two quantities that are related inversely,

For example,

The time it takes to cover a particular distance will decrease if the speed of a vehicle(bike) increases

Extramarks provides different study material, solutions to the problems, revision notes, sample papers, and previous year’s question papers that help students to a great extent.

• Revision Notes of class 8 chapter 13 Mathematics Direct, and Inverse Proportions are available online for students in PDF format, with solved answers on Extramarks’official website and mobile app.
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### About Direct and Inverse Proportion

We may see some direct or indirect relationship between two things in everyday life, like

• The bill amount to pay, will increase if the number of things purchased increases. (direct relationship).
• The time it will take to reach a destination will decrease if the speed of the car increases. (indirect relationship).

According to Mathematics, any two physical quantities that vary are said to be proportional if they are multiplicatively related to each other by a constant term. For example, If we eat more, we will gain more energy, and

In Mathematics, there exist two kinds of proportionalities.

When one quantity increases along with the other quantity, it is called directly proportional, whereas if one quantity increases and the other quantity decreases, it is called inversely proportional.

• Direct proportion: It means that the ratio of the value of two quantities is constant at any instant. If the number of one increases, the quantity of the other will also increase. For example, if we run more, we will lose more energy.
• Inverse proportion: It means that the product of the values of two quantities is a constant at any instant. If the number of one increases, the quantity of the other will decrease. Example: Our physical fitness will reduce if we eat more junk food.

These CBSE revision notes are made according to the CBSE syllabus. The questions in these notes are based on CBSE’s previous year’s question papers. These notes contain important questions and formulas to help students achieve better marks in their examinations. These notes also contain CBSE extra questions that will help students test their understanding.

## Difference Between Direct and Inverse Proportion

 Direct proportion Inverse Proportion When two measurable quantities differ from each. When two measurable quantities are related inversely. If the quantity of one increases, the quantity of the other will also increase. If the quantity of one increases, the quantity of the other will decrease. If the quantity decreases, the quantity of others will also decrease. If the quantity of one decreases, the quantity of the other will increase. The ratio of the value of two quantities is constant at any instant. The product of the values of two quantities is a constant at any instant For example, the amount of work done will increase if the amount of force applied increases. For example, the pressure will increase if the volume of an object decreases.

### Symbol Of Proportion

If two quantities, say a and b are in proportion, then they are written as a ∝ b, where ∝ represents “is proportion to”.

Two methods exist to solve the problems related to direct proportion-

1. Tabular Method
2. Unitary Method

### Why Choose Extramarks for Direct and Inverse Proportion Revision Notes?

Mathematician experts at Extramarks have made these revision notes to cover all the important concepts, equations, formulas, and solved examples. Extramarks chapter 13 Mathematics class 8 notes will enhance the student’s confidence level, enabling them to perform better in their final exams.

The Extramarks’ Revision Notes are based on the updated CBSE syllabus. Highly qualified teachers at Extramarks focus on the important topics from the exam point of view and highlight them. These notes contain all the topics in the NCERT books concisely. Download the Class 8 Mathematics Chapter 5 notes on this page to improve your exam results. Once you examine these notes, you can easily solve CBSE sample papers to test your understanding. These notes also contain CBSE extra questions that will help students test their understanding.

### 1. What do you mean by Direct Proportion?

If the relation between two quantities is where the ratio of the two is equal to a constant value, it is called direct proportion or direct variation.

### 2. What do you mean by Inverse Proportion?

When one value increases and the other decreases are known as an Inverse proportion.

### 3. How can time and work problems be used?

Time and work problems are important as there is a relationship between the number of persons doing the work, the number of days or time taken to complete the work, and the amount of work done.

### 4. Here are the car parking charges near a railway station: 4 hours-Rs.60 8 hours-Rs.100 12 hours-Rs.140 24 hours-Rs.180 Find out if the parking charges are directly proportional to the parking time.

Charges per hour will be:

C1 =604 = Rs. 15

C2 =1008 = Rs. 12.50

C3 =14012 = Rs. 11.67

C4 =18024 = Rs.7.50

Here, the charges per hour are not the same, i.e., C1 ≠ C2 ≠ C3 ≠ C4

So, we can conclude that the parking charges are not in direct proportion to the parking time.