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

I haven't heard of that rule either. Kinda neat.

I wouldn't teach that in a math class unless I was at a spot where the goal
was to demonstrate how a general equation can be used to apply a shortcut.

I wouldn't teach that for the same reason I avoid things like the triangle
trick for algebra and subscript/charge swapping in creating chemical
formulas. Students learn the trick, but often miss the content they are
supposed to be learning.

By triangle trick I mean
d
v * t
cover the variable you are attempting to solve for and the remaining
arrangement is the equation you need. Cover d --> d = v*t; cover t --> t =
d/v

The charge subscript swapping:
Ca+2 + F-1 [swap charge for subscript --> neutral formula = CaF2
Kids can create neutral compounds, but have no underlying understanding of
what is happening.

Have a good one.
Paul.

Paul Lulai
Physics Teacher
St Anthony Village Senior High
St Anthony Village MN 55418

On Wed, Nov 21, 2018 at 8:29 AM Carl Mungan <mungan@usna.edu> wrote:

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)
Naval Academy Stop 9c, 572C Holloway Rd, Annapolis MD 21402-1363
mailto:mungan@usna.edu http://usna.edu/Users/physics/mungan/

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