Re: [Phys-L] Rule of 9's?

[Carl Mungan <mungan@usna.edu>]

I’ve never heard of the rule either. I don’t see much virtue to including the absolute value:

AB - BA = (10A+B) - (10B+A) = 9(A-B) which works for *any* A and B, such as the one in your example.

My question is whether this rule is particularly useful: If someone cannot subtract two-digit numbers confidently, are they going to succeed at doing this alternative which requires doing and keeping track of two other operations?

> On Nov 21, 2018, at 8:31 AM, Peter Schoch <pschoch@fandm.edu> wrote:
>
> Hi,
>
>
>
> I was helping my daughter with her 8th grade  math last night, and she was
> marveling at my ability to do so much in my head.  I told her that it was
> practice, and some ‘tricks’.  (I didn’t mention that I thought it might
> also be that she relies on a calculator too much.)
>
>
>
> The one trick I showed her that totally baffled her was what we used to
> call the ‘Rule of 9’s’:  If you have a 2 digit number, AB, and are
> subtracting off a number, BA, the result is just abs(A-B)*+/-9.  It takes
> longer to write it out than to do it in your head.
>
>
>
> Example 19 -91 =?
>
> Abs(9-1)*-9=-72
>
>
>
> She was amazed by this, and wondered why her teachers had never shown her
> this.  Then, she asked me if there was a name for this and who thought of
> it.  This is where I am stumped, is there a ‘real’ name for this?  I don’t
> think “Rule of 9’s” is a true name.  Also, do you know who might of first
> thought of this?  (I asked my colleagues in the math dept. and they had
> never heard of this.)
>
>
>
> Thanks,
> Peter
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-----
Carl E. Mungan, Professor of Physics  410-293-6680 (O) -3729 (F)
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