We will learn the subtraction of rational number with different denominator. To find the difference of two rational numbers which do not have the same denominator, we follow the following steps:
Step I: Let us obtain the rational numbers and see whether their denominators are positive or not. If the denominator of one (or both) of the numerators is negative, rearrange it so that the denominators become positive.
Step II: Obtain the denominators of the rational numbers in step I.
Step III: Find the lowest common multiple of the denominators of the two given rational numbers.
Step IV: Express both the rational numbers in step I so that the lowest common multiple of the denominators becomes their common denominator.
Step V: Write a rational number whose numerator is equal to the difference of the numerators of rational numbers obtained in step IV and denominators is the lowest common multiple obtained in step III.
Step VI: The rational number obtained in step V is the required difference (simplify if required).Following examples will illustrate the above procedure.
1. Subtract 9 from 4/5
Solution:
We have, 9 = 9/1
Clearly, denominators of the two rational numbers are positive. We now rewrite them so that they have a common denominator equal to the LCM of the denominators.
In this case the denominators are 1 and 5.
The LCM of 1 and 5 is 5.
We have, 9 = 9/1 = 9 × 5/1 × 5 = 45/5
Therefore, 4/5  9
= 4/5  9/1
= 4/5  45/5
= (4  45)/5
= 41/5
Therefore, 4/5  9 = 41/5
2. Find the difference of: 3/4  5/6
Solution:
The denominators of the given rational numbers are 4 and 6 respectively.
LCM of 4 and 6 = (2 × 2 × 3) = 12.
Now, 3/4 = (3) × 3/4 × 3 = 9/12
and 5/6 = 5 × 2/6 × 2 = 10/12
Therefore, 3/4  5/6
= 9/12  10/12
= (9  10)/12
= 19/12
Therefore, 3/4  5/6 = 19/12
3. Simplify: 3/15  7/12
Solution:
First we write each of the given numbers with positive denominator.
3/15 = 3 × (1)/(15) × (1) = 3/15, [Multiplying the numerator and denominator by 1]
⇒ 3/15 = 3/15
7/12 = 7 × (1)/(12) × (1) = 7/12, [Multiplying the numerator and denominator by 1]
⇒ 7/12 = 7/12
Therefore, 3/15  7/12 = 3/15  (7)/12
Now, we find the LCM of 15 and 12.
The LCM of 15 and 12 = 60
Rewriting 3/15 in the form in which it has denominator 60, we get
3/15 = 3 × 4/15 × 4 = 12/60
Rewriting 7/12 in the form in which it has denominator 60, we get
7/12 = 7 × 5/12 × 5 = 35/60
Therefore, 3/15  7/12
= 3/15  (7)/12
= 12/60  (35)/60
= (12)  (35)/60
= 12 + 35/60
= 23/60
Thus, 3/15  7/12 = 23/60.
4. Simplify: 11/18  5/12
Solution:
First we write each one of the given rational numbers with positive denominator.
Clearly, denominator of 5/12 is positive.
The denominator of 11/18 is negative.
The rational number 11/18 with positive denominator is 11/18.
Therefore, 11/18  5/12 = 11/18  5/12
The LCM of 18 and 12 is 36.
Rewriting 11/18 in forms having the same denominator 36, we get
11/18 = (11) × 2/18 × 2, [Multiplying the numerator and denominator by 2]
⇒ 11/18 = 22/36
Rewriting 5/12 in forms having the same denominator 66, we get
5/12 = 5 × 3/12 × 3, [Multiplying the numerator and denominator by 3]
⇒ 5/12 = 15/36
Therefore, 11/18  5/12
= 11/18  5/12
= 22/36  15/36
= 22  15/36
= 37/36
Therefore, 11/18  5/12 = 37/36
If a/b and c/d are two rational numbers such that b and d do not have a common factor other than 1, i.e., HCF of b and d is 1, then
a/b  c/d = a × d  c × b/b × d
For example, 5/18  3/13 = 5 × 13  3 × 18/18 × 13 = 65  54/234 = 11/234
and 2/11  3/14 = (2) × 14  (3 × 11)/11 × 14 = 28  33/154 = 61/154
● Rational Numbers
Introduction of Rational Numbers
Is Every Rational Number a Natural Number?
Is Every Rational Number an Integer?
Is Every Rational Number a Fraction?
Equivalent form of Rational Numbers
Rational Number in Different Forms
Properties of Rational Numbers
Lowest form of a Rational Number
Standard form of a Rational Number
Equality of Rational Numbers using Standard Form
Equality of Rational Numbers with Common Denominator
Equality of Rational Numbers using Cross Multiplication
Comparison of Rational Numbers
Rational Numbers in Ascending Order
Rational Numbers in Descending Order
Representation of Rational Numbers on the Number Line
Rational Numbers on the Number Line
Addition of Rational Number with Same Denominator
Addition of Rational Number with Different Denominator
Properties of Addition of Rational Numbers
Subtraction of Rational Number with Same Denominator
Subtraction of Rational Number with Different Denominator
Subtraction of Rational Numbers
Properties of Subtraction of Rational Numbers
Rational Expressions Involving Addition and Subtraction
Simplify Rational Expressions Involving the Sum or Difference
Multiplication of Rational Numbers
Properties of Multiplication of Rational Numbers
Rational Expressions Involving Addition, Subtraction and Multiplication
Reciprocal of a Rational Number
Rational Expressions Involving Division
Properties of Division of Rational Numbers
Rational Numbers between Two Rational Numbers
8th Grade Math Practice
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