Difference between revisions of "Divisibility rules/Rule for 2 and powers of 2 proof"
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== Proof == | == Proof == | ||
+ | === Basic Idea === | ||
+ | Let the number <math>N</math> be <math>(10^n)k + p</math> where k and p are integers and <math>p<(10^n)</math>. Since <math>\frac{10^n}{2^n}</math> is <math>5^n</math>, <math>(10^n)</math> is a multiple of <math>2^n</math>, meaning <math>(10^n)k</math> is also a multiple of <math>2^n</math>. As long as p is a multiple of <math>2^n</math>, then <math>N</math> is a multiple of <math>2^n</math>. Since <math>(10^n)k</math> has <math>n</math> trailing 0's, <math>p</math> is the last <math>n</math> digits of the number <math>n</math>. | ||
+ | |||
+ | === Concise === | ||
''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.'' | ||
− | Let <math>N | + | 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> | |
− | Taking <math> | + | Taking this <math>\mod 2^n</math> we have |
− | {| class="wikitable" style="margin: 1em auto 1em auto | + | {| class="wikitable" style="margin: 1em auto 1em auto" |
− | | <math>N</math> || <math>= 10^k a_k + 10^{k-1} a_{k-1} + \cdots + 10 a_1 + a_0</math> | + | | <math>N</math> || <math>= 10^k a_k + 10^{k-1} a_{k-1} + \cdots + 10 a_1 + a_0 </math> |
|- | |- | ||
− | | || <math> | + | | || <math>\equiv 10^{n-1} a_{n-1} + 10^{n-2} a_{n-2} + \cdots + 10 a_1 + a_0 \pmod{2^n}</math> |
− | |||
− | |||
|} | |} | ||
+ | |||
+ | because for <math>i \geq n</math>, <math>10^i \equiv 0 \pmod{2^n}</math>. Thus, <math>N</math> is divisible by <math>2^n</math> if and only if | ||
+ | |||
+ | <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>2^n</math> if and only if <math>N</math> is divisible by <math>2^n</math>. | ||
== See also == | == See also == | ||
* [[Divisibility rules | Back to divisibility rules]] | * [[Divisibility rules | Back to divisibility rules]] | ||
+ | [[Category:Divisibility Rules]] |
Latest revision as of 16:46, 3 May 2020
A number is divisible by if the last digits of the number are divisible by .
Contents
Proof
Basic Idea
Let the number be where k and p are integers and . Since is , is a multiple of , meaning is also a multiple of . As long as p is a multiple of , then is a multiple of . Since has trailing 0's, is the last digits of the number .
Concise
An understanding of basic modular arithmetic is necessary for this proof.
Let the base-ten representation of be where the are digits for each and the underline is simply to note that this is a base-10 expression rather than a product. If has no more than digits, then the last digits of make up itself, so the test is trivially true. If has more than digits, we note that:
Taking this we have
because for , . Thus, is divisible by if and only if
is. But this says exactly what we claimed: the last digits of are divisible by if and only if is divisible by .