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Difference between revisions of "Divisibility rules/Rule for 5 and powers of 5 proof"
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''An understanding of [[Introduction to modular arithmetic | basic modular arithmetic]] is necessary for this proof.''
 
''An understanding of [[Introduction to modular arithmetic | basic modular arithmetic]] is necessary for this proof.''
  
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Let the [[base numbers | base-ten]] representation of <math>N</math> be <math>\overline{a_ka_{k-1}\cdots a_1a_0}</math> where the <math>a_i</math> are digits for each <math>i</math> and the bar is simply to note that this is a base-10 expression rather than a product.  If <math>N</math> has no more than <math>n</math> digits, then the last <math>n</math> digits of <math>N</math> make up <math>N</math> itself, so the test is trivially true.  If <math>N</math> has more than <math>n</math> digits, we note that:  
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Let the [[base numbers | base-ten]] representation of <math>N</math> be <math>\underline{a_ka_{k-1}\cdots a_1a_0}</math> where the <math>a_i</math> are digits for each <math>i</math> and the underline is simply to note that this is a base-10 expression rather than a product.  If <math>N</math> has no more than <math>n</math> digits, then the last <math>n</math> digits of <math>N</math> make up <math>N</math> itself, so the test is trivially true.  If <math>N</math> has more than <math>n</math> digits, we note that:  
  
 
<center><math> N = 10^k a_k + 10^{k-1} a_{k-1} + \cdots + 10 a_1 + a_0. </math></center>
 
<center><math> N = 10^k a_k + 10^{k-1} a_{k-1} + \cdots + 10 a_1 + a_0. </math></center>
Line 18: Line 18:
 
because for <math>i \geq n</math>, <math>10^i \equiv 0 \pmod{5^n}</math>.  Thus, <math>N</math> is divisible by <math>5^n</math> if and only if  
 
because for <math>i \geq n</math>, <math>10^i \equiv 0 \pmod{5^n}</math>.  Thus, <math>N</math> is divisible by <math>5^n</math> if and only if  
  
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<center><math>10^{n-1} a_{n-1} + 10^{n-2} a_{n-2} + \cdots + 10 a_1 + a_0 = \overline{a_{n-1}a_{n-2}\cdots a_1a_0}</math></center>
+
<center><math>10^{n-1} a_{n-1} + 10^{n-2} a_{n-2} + \cdots + 10 a_1 + a_0 = \underline{a_{n-1}a_{n-2}\cdots a_1a_0}</math></center>
  
 
is.  But this says exactly what we claimed: the last <math>n</math> digits of <math>N</math> are divisible by <math>5^n</math> if and only if <math>N</math> is divisible by <math>5^n</math>.
 
is.  But this says exactly what we claimed: the last <math>n</math> digits of <math>N</math> are divisible by <math>5^n</math> if and only if <math>N</math> is divisible by <math>5^n</math>.

Latest revision as of 09:58, 17 August 2006

A number $N$ is divisible by $5^n$ if the last $n$ digits are divisible by that power of 5.

Proof

An understanding of basic modular arithmetic is necessary for this proof.

Let the base-ten representation of $N$ be $\underline{a_ka_{k-1}\cdots a_1a_0}$ where the $a_i$ are digits for each $i$ and the underline is simply to note that this is a base-10 expression rather than a product. If $N$ has no more than $n$ digits, then the last $n$ digits of $N$ make up $N$ itself, so the test is trivially true. If $N$ has more than $n$ digits, we note that:

$N = 10^k a_k + 10^{k-1} a_{k-1} + \cdots + 10 a_1 + a_0.$

Taking this $\mod 5^n$ we have

$N$ $= 10^k a_k + 10^{k-1} a_{k-1} + \cdots + 10 a_1 + a_0$
$\equiv 10^{n-1} a_{n-1} + 10^{n-2} a_{n-2} + \cdots + 10 a_1 + a_0 \pmod{5^n}$

because for $i \geq n$, $10^i \equiv 0 \pmod{5^n}$. Thus, $N$ is divisible by $5^n$ if and only if

$10^{n-1} a_{n-1} + 10^{n-2} a_{n-2} + \cdots + 10 a_1 + a_0 = \underline{a_{n-1}a_{n-2}\cdots a_1a_0}$

is. But this says exactly what we claimed: the last $n$ digits of $N$ are divisible by $5^n$ if and only if $N$ is divisible by $5^n$.

See also

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