 # Using Factor Trees to find the least common multiple (LCM)

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Using Factor Trees to find the least common multiple (LCM)

Least Common Multiple The least common multiple of two (or more) numbers is the smallest number that can be divided by both (or all) the numbers. Example the LCM of 4 and 7 is 4x7 = 28

Finding the LCM To find the LCM we can list all the multiples of each number and find the first one that appears in each list Eg LCM 24 and 36 24, 48, 72, 96, 36, 72, 108, So the LCM of 24 and 36 is 72

But, if you have big/nasty numbers
This is takes ages Easy to make a mistake

LCM – Using Factor Trees

Find the LCM of 45 and 72 First write each number as a product of its primes Find the Highest Common Factor by identifying the primes which appear in both numbers Find the Lowest Common Multiple by multiplying the Highest Common Factor by all the factors that are leftover. HELP!!

1. Product of Primes 45 72 3 x 3 x 5 = 45 2 x 2 x 2 x 3 x 3 = 72
36 5 9 2 18 3 3 2 9 3 x 3 x 5 = 45 32 x 5 = 45 2 x 2 x 2 x 3 x 3 = 72 23 x 32 = 72 3 3

2. Highest Common Factor HCF = 3 x 3 = 9 45 72 72 45 2 36 5 9 2 18 3 3

3. LCM = HCF x Leftovers HCF = 3 x 3 = 9 Leftovers = 5 x 23 = 40
45 45 3 9 5 72 72 2 36 18 3 9 HCF = 3 x 3 = 9 Leftovers = 5 x 23 = 40 LCM = 9 x 40 = 360

Find the HCF and then LCM of the following:
90 108 70 165 168 210 1400 756

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