Re: 'mode' for a scene?
- From: nyra <nyra@xxxxxxx>
- Date: Wed, 05 Oct 2005 21:53:19 +0200
Catja Pafort schrieb:
>
> Michael R N Dolbear <me@xxxxxxxxxxx> wrote:
>
> > 237 is 'obviously' divisible by 3,
>
> I knew the multiply-by-eleven rule, but why is this one obvious?
Deliberately super-terse: because we're working in base 10.
For every base, calculating the sum of the digits allows you to check
for divisibility of [base minus one] and all its prime factors - in
hexadecimal, it allows you to check for 3, 5 and 15; in octal for 7.
We were told this summing up in third or fourth grade, i think - as a
method to do a quick check. The teacher started by writing numbers
with 6+ digits on the blackboard and asking whether the number was
divisible by three or not. In spite of not knowing the trick, i was
the first to answer for several of them, simply by brute force.
--
Een koe is een merkwaardig beest; wat er ook in haar geest moge zijn,
haar laatste woord is altijd boe.
.
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