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] |

*From*: "Lulai, Paul" <plulai@stanthony.k12.mn.us>*Date*: Wed, 21 Nov 2018 09:46:24 -0600

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:was

Hi,

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 wasdon’t

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

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

_______________________________________________

Forum for Physics Educators

Phys-l@mail.phys-l.org

http://www.phys-l.org/mailman/listinfo/phys-l

-----

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/

_______________________________________________

Forum for Physics Educators

Phys-l@mail.phys-l.org

http://www.phys-l.org/mailman/listinfo/phys-l

**References**:**[Phys-L] Rule of 9's?***From:*Peter Schoch <pschoch@fandm.edu>

**Re: [Phys-L] Rule of 9's?***From:*Carl Mungan <mungan@usna.edu>

- Prev by Date:
**Re: [Phys-L] Rule of 9's?** - Next by Date:
**Re: [Phys-L] ?simple experiment limited by fluctuations** - Previous by thread:
**Re: [Phys-L] Rule of 9's?** - Next by thread:
**[Phys-L] ASU MNS in Physics for HS Teachers** - Index(es):