# Difference between revisions of "Divisibility rules/Rule for 2 and powers of 2 proof"

m (→Proof) |
(Added category) |
||

Line 24: | Line 24: | ||

== See also == | == See also == | ||

* [[Divisibility rules | Back to divisibility rules]] | * [[Divisibility rules | Back to divisibility rules]] | ||

+ | [[Category:Divisibility Rules]] |

## Revision as of 19:26, 6 March 2014

A number is divisible by if the last digits of the number are divisible by .

## Proof

*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 .