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Q&A #494

Teachers' Lounge Discussion: Multiplication tables

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From: Joan <whitefairy75@yahoo.com>
To: Teacher2Teacher Public Discussion
Date: 2005030313:06:02
Subject: Re: Re: Re: multiplication



Hi,


Loyd.... and all those who are particularly interested in this type of
Math....


Note that I told you this type of multiplication requires you to learn
all the tables upto 5x5.

So 5x4 is before that!!!

Although you caught me there with a surprise....

Any way there are many more tricks just like this. 

I got this from a book in my college library when I was a student.

Now I am going to give you the most close algebraic explanation for
this...

(x-a)(x-b)=x(x-a-b)+ab

A slight difference , however,is noticeable when the vertical
multiplication of the deficit digits(for obtaining the right hand side
portion of the answer) yeilds a product consisting of more than one
digit.

For example:7x6

       7  /3
       6  /4
       ______
       3  /(1)2

The required vertical multiplication of the numbers 3 and 4 yields 12.
As our base is 10  (10-7=3 and 10-6=4), we need only one digit in the
one's place. So that 1 is added onto the 3.

But that is what we always do ... when we have a carry we add it to
the next place!!!!

About the 3, 4 or 5th grade child understanding this...

I have been teaching this for the 2nd grade when I started out with
teaching multiplication tables for them in India. It did work. (We
have to cover the tables upto 12x12 in 2nd grade in India). The only
thing is never tell them this to be an easy way to completely avoid
tables!!! I taught them this as a game.

I have reached U.S only on 2004 July end and as my daughter will go to
K.G only next year, I am knowing many of the changes in syllabus in
India and US. So you can always adjust it with the standard of the
children in your class. My daughter already knew counting 1 to 100 and
had started to learn the spellings of three letters back in India
itself.(She was 3+ and in the pre-nursery there). So the primary
classes are a little ahead of those in US. 

I do know many of these simple math magics which I got to learn from 
books back in India. Although we don't follow them in the school
curriculum, they are simply kept as reference books wherein there are
tricky methods even to solve calculus problems!!! 

I feel very sorry myself for not bringing the book over here.

Joan
 

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