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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
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Carl E. Mungan, Professor of Physics 410-293-6680 (O) -3729 (F)
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