Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

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
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 =

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

On Wed, Nov 21, 2018 at 8:29 AM Carl Mungan <> 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

On Nov 21, 2018, at 8:31 AM, Peter Schoch <> wrote:


I was helping my daughter with her 8th grade math last night, and she
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 =?


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
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.)

Forum for Physics Educators

Carl E. Mungan, Professor of Physics 410-293-6680 (O) -3729 (F)
Naval Academy Stop 9c, 572C Holloway Rd, Annapolis MD 21402-1363

Forum for Physics Educators