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### Least Common Multiple

```
Date: 10/26/1999 at 13:56:34
From: Michelle Boyce
Subject: Common Numbers

What is the lowest number that is divisible by the numbers 1 through
10? Someone told me that if I take away 1, 2, 3, 4, 5, 6 and multiply
7 x 8 x 9 x 10 = 5040. I don't understand how they figured that out
and what the term for that kind of math is called. Can you help me? I
need someone to explain the steps to me.
```

```
Date: 10/26/1999 at 16:15:41
From: Doctor Rob
Subject: Re: Common Numbers

Thanks for writing to Ask Dr. Math, Michelle.

You are asking for the Least Common Multiple of 1, 2, 3, 4, 5, 6, 7,
8, 9, and 10. Your "somebody" is not correct. The number 5040 is a
multiple of all of them, but it is not the least such.

To solve such problems, you can write the prime-power factorization of
each of the numbers, and the exponent of the prime powers that occur:

exponent of
number  factorization    2  3  5  7
------  -------------    -  -  -  -
1          1          0  0  0  0
2          2^1        1  0  0  0
3          3^1        0  1  0  0
4          2^2        2  0  0  0
5          5^1        0  0  1  0
6          2^1*3^1    1  1  0  0
7          7^1        0  0  0  1
8          2^3        3  0  0  0
9          3^2        0  2  0  0
10          2^1*5^1    1  0  1  0

Now take the maximum in each column, and use those as exponents for

3  2  1  1

2^3 * 3^2 * 5^1 * 7^1

- Doctor Rob, The Math Forum
http://mathforum.org/dr.math/
```
Associated Topics:
High School Number Theory
Middle School Division

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