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

On 11/21/18 7:29 AM, Carl Mungan wrote:

I’ve never heard of the rule either.

It's new to me, too.

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,

Agreed.

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

I like it.

a) It's cute.

b) It's useful as a check on other methods.

c) It generalizes to three-digit numbers:
ABC-CBA = 99(A-C)
= 100(A-C) - (A-C)