More Lowest Common Multiples
Y7
Previously we looked at listing multiples to find the LCM (lowest common multiple).
Find the LCM of 2, 3 and 4.
Multiples of 2 :
Multiples of 3:
Multiples of 4:
2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26...
3, 6, 9, 12, 15, 18, 21, 24, 27....
4, 8,
12,
16,
20,
24,
28.......
Notice a patttern of the common multiples 12, 24, 36,.. The lowest common multiple being 12.
LCM of 2, 3 and 4 = 12
Using Prime Factors
The prime factor 3 is common.
Use only one of them in the
multiplication to find the LCM.
Find the LCM of 9 and 12.
Find the prime factors.
9 = 3×3
12 = 3 × 2 × 2
Find the LCM of 18 and 60.
Find the prime factors.
18 = 3 × 3 × 2
60 = 5 × 3 × 2 × 2
A.
Use this method to find the LCM of
1).
5).
3 and 4
15 and 20
LCM of 9 and 12 = 3 × 3 × 2 × 2 = 36
The prime factor 3 is common.
Use only one of them in the
The prime factor 2 is common.
multiplication to find the LCM.
Use only one of them in the
multiplication to find the LCM.
2).
6).
6 and 9
15 and 21
3).
7).
LCM of 18 and 60 = 3 × 3 × 2 × 5 × 2 = 180
10 and 12
3, 4 and 8
4).
8).
15 and 24
5, 8 and 15.
Repeated division by Prime Factors
Find the LCM of 9 and 12.
3 9, 12
3, 4
3 9, 12
3 3, 4
1, 4
Divide both numbers by
a prime factor. Write the
answers in the row below.
If a number is not divisible
by a factor, carry it down to
the next row.
B.
Use this method to find the LCM of
1).
5).
9).
4 and 7
12 and 21
4, 5 and 10
3 9, 12
3 3, 4
2 1, 4
1, 2
2).
6).
10).
3
3
2
2
Sometimes you will be
dividing just one of the
numbers.
9, 12
3, 4
1, 4
1, 2
1, 1
Stop dividing when all the
numbers become 1.
LCM of 9 and 12 = 3 × 3 × 2 × 2 = 36
3 and 5
6 and 14
3, 8 and 12
3).
7).
11).
4 and 10
12 and 18
3, 4 and 15
4).
8).
12).
6 and 8
16 and 24
8, 12 and 20.
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