Question
Find Least Common Multiple of 21, 31 and 42 using prime factorisation method.
Answer
LCM( 21, 31, 42 ) = 1302
Explanation
Step 1: Write down factorisation of each number:
21 = 3 · 7
31 = 31
42 = 2 · 3 · 7
Step 2 : Match primes vertically:
| 21 | = | 3 | · | 7 | ||||
| 31 | = | 31 | ||||||
| 42 | = | 2 | · | 3 | · | 7 |
Step 3 : Bring down numbers in each column and multiply to get LCM:
| 21 | = | 3 | · | 7 | ||||||
| 31 | = | 31 | ||||||||
| 42 | = | 2 | · | 3 | · | 7 | ||||
| LCM | = | 2 | · | 3 | · | 7 | · | 31 | = | 1302 |
This solution can be visualized using a Venn diagram.
The LCM is equal to the product of all the numbers on the diagram.
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