Rational number for class 8 notes

Introduction

Till now, we have studied the natural number, whole number, integers, decimal, and fraction. Now in class 8, we will study a new type of number which is called” rational number”.Here is the complete Rational number class 8 notes explanation which will help you to prepare all your concepts regarding this chapter. If you find any DOUBT regarding any topic COMMENT BELOW without any hesitation. I am here to help you

In this rational number class 8 notes, you will learn these topics.

  • Rational number 
  • Properties of rational numbers: closure property, commutative property, associative property, and distributive property.
  • Additive identity and multiplicative identity
  • Additive inverse and multiplicative inverse 
  • Rational numbers between two rational number.
  • word problem

What is a rational number?

The numbers which can be written in form of p/q , where p and q are integers and q≠0 are called rational numbers.

Example:0/1,3/3,-8/-2……etc

Now ,you may think why q ≠0?

It is because if the value of q will zero i.e q=0 then, P/q will become P/0 =∞ which is not defined

that is why the value of q cannot be zero but p can be zero because 0/q =0 which is defined.

Notes:

  • Zero is a rational number
  • Every integer is a rational number 
  • Natural number, whole number, integers, and decimals numbers are not in form of p/q but can be written in the form p/q and this is why these numbers are also rational numbers.
  • Every fraction is a rational number

Equivalent rational numbers

If p/q is any rational number and m is a non-integer(cannot be zero) then

\frac{p}{q}=\frac{p×m}{q×m} is called an equivalent rational number.

e.g         If 5/6 is a rational number and 3 is an integer, then

\frac{5×3}{6×3}=\frac{15}{18} is the equivalent rational number

We can form a lot of equivalent rational numbers from any single rational number.In the above example 15/18 is an equivalent rational number which formed due to multiplication by 3. Like this, we can form more numbers by multiplication of 3,4,5 etc. I think, now it is clear to you.

The standard form of a rational number 

A rational number p/q is said to be in standard form if p and q have no common divisor other than 1. In simple language, the simplest of any rational number(equivalent rational number) is called the standard form of a rational number. Let us understand it with an example.

e.g. We have 15/18 as an equivalent rational number when we divide by 3 in the numerator and denominator it becomes 5/6 which is the least form of 15/18. So 5/6 is the standard form of rational number 15/18

Another way to convert it through HCF of 15 and 18. The HCF of 15 and 18 is 3, when we divide numerator and denominator by 3, we will get the same result. Now, you will read about the properties of rational numbers further in this rational number class 8 notes.

Properties of the rational number

1.Closure property

we have 5/6 and 3/5 as a rational number

Addition

5/6+3/5=43/30 which is a rational number

subtraction

5/6-3/5 =7/30 which is rational number

Multiplication

5/6×3/5=15/30 which is a rational number

Division

5/6÷3/5=25/18 which is a rational number

Rational number are closed under addition, subtraction, multiplication, and division (excluding division by zero)

2.Commulative property

Addition

1/2+4/5=13/10

4/5 +1/2=13/10

so,  1/2+4/5=4/5 +1/2

Subtraction

Is       5/7-3/7=3/7-5/7

you will observe that  5/7-3/7≠3/7-5/7

Multiplication

3/8×5/7=15/56

5/7×3/8=15/56

3/8×5/7=5/7×3/8

Division

Is 1/2÷-3/4=-3/4÷1/2?

you will observe that 1/2÷-3/4≠-3/4÷1/2

So, Commutative property holds for addition and multiplication of rational numbers only . This property does not hold for subtraction and division of rational numbers

3.Associative property

Addition

1/2+(-4/5+2/3)=11/30                                          using a+(b+c)=(a+b)+c

(1/2+-4/5)+2/3=11/30

1/2+(-4/5+2/3)=(1/2+-4/5)+2/3

Subtraction

Is -1/3-(6/5-2/3)=(-1/3-6/5)-2/3)  ?                 using a-(b-c)=(a-b)-c

you will observe that -1/3-(6/5-2/3)=(-1/3-6/5)-2/3)

Multiplication

(5/3×-3/4)×1/2=-15/56                                       using a×(b×c)=(a×b)×c

5/7×(-3/4×1/2) =-15/56

hence,(5/3×-3/4)×1/2=5/7×(-3/4×1/2)

Division                                                                      using a÷(b÷c)=(a÷b)÷c

Is 4/5÷(6/5÷3/2)=(-4/5÷6/5)÷3/2?

You will observe that 4/5÷(6/5÷3/2)=(-4/5÷6/5)÷3/2?

Note: Associative property is also called Re-arrangement property

4.Distributive property of multiplication over addition for rationals number

4/5×(2/3+7/15)=4/5×(10+7/15) =4/5×17/15=68/75                using a×(b+c)=a×b+b×c

4/5×2/3+4/5×7/15=68/75

thus,4/5×(2/3+7/15)=4/5×2/3+4/5×7/15

Additive identity and multiplicative identity

Before learning about these you should be familiar with identity check this 

Additive identity: The adding result which is true for all numbers is called additive identity. So 0 is an only additive identity because when we add 0 to any number the result will always the same as that number

e.g     5+0=5  ,   -3+0=-3

Multiplicative identity:  The multiplication result which is true for all numbers is called multiplicative identity. Here 1 is a multiplicative identity because when we multiply any number by 1 the result will be the same number.

e.g      5×1=5   ,-3×1=-3

Rational number class 8 notes:[Additive inverse and multiplicative inverse]

Now we will discuss two very important terms “additive inverse ” and “multiplicative inverse” in this rational number notes . First, start with additive inverse

Additive inverse: Additive inverse of any number is the opposite number which when add to the number gives 0 as a result.

e.g   The additive inverse of 5 is -5.so 5+(-5)=0 result is zero.

The additive inverse of -5 is 5.so -5+(5)=0 result is zero.

Multiplicative inverse: Multiplicative inverse of any number is the number which when multiplied to the original number gives 1 as a result.

e.g     The multiplicative inverse of 2/3 is 3/2.2/3×3/2=1 as a result

Notes

  • The additive inverse of zero is zero.
  • 1 and-1 are a rational number which is their own reciprocals

Rational number class 8 notes:[ Rational number between two number]

There are countless rational numbers between two rational numbers. It is not possible to find all the rational numbers but we can find how many we want. There are two methods to find a rational number between two numbers. Let us understand them one-by-one

Q.1 Find three rational numbers lying between 1/3 and 1/2.

solution: first method

step.1  Take the Lcm of both denominators i.e Lcm of 3 and 2 is 6

step.2    Make the denominator of fractions 1/3 and 1/2 by 2 and 3 respectively.

step.3   After multiplication, the fraction will 2/6 and 3/6

step.4   Multiply by 10 in both fraction’s numerator and denominator, it become 20/60 and 30/60

step.5 Now we can easily find ten rational numbers between these i.e 21/60,22/60,23/60,24/60………….29/60

step.6 we can choose any three rational numbers among these. If we want find more than 10 rational number then, again multiply by 10.

Second method 

This method also known as the arithmetic mean method. We find the arithmetic mean of two given rational numbers to find the mid-value of the rational numbers. Let understand it with the help of the above examples

We have \frac{1}{3} and \frac{1}{2} are two rational number .The mid-value of these numbers will be \frac{\frac{1}{3} +\frac{1}{2}}{2}=\frac{5}{12}.Similarly ,You can find more arithmetic mean of numbers such as \frac{5}{12} and \frac{1}{2}.I think ,now it is clear . But I will recommend you to solve problem of this type by first method . That method is very simple and fast .

Rational number class 8 notes :[Word problem]

1)  The product of two rational numbers is \frac{-56}{25}.If one number is \frac{-8}{15},Find the other

Solution:

Product of two rational numbers=\frac{-56}{25}

One number is given=\frac{-8}{15}

Let the other number will=x

A/Q

\frac{-8}{15}×x=\frac{-56}{25}

x=\frac{21}{5}

2) Using distributive property, evaluate \frac{-5}{3}×\frac{5}{7}-\frac{4}{7}×\frac{5}{3}

Solution:

\frac{-5}{3}×\frac{5}{7}-\frac{4}{7}×\frac{5}{3}

\frac{-5}{3}×(\frac{5}{7}+\frac{4}{7})

\frac{-5}{3}×\frac{9}{7}

=\frac{-15}{7} is the answer

 

 

 

 

 

 

 

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