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## #1 2011-05-04 21:45:06

123ronnie321
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### Cool way to square multiples of five

Think of any number that is a multiple of five.
Eg. - 65.
65 is between 60 and 70. So multiply them in your mind 670=4200. Attach 25 in the end.
∴65²= 4225.

Can you guess why it works?

## #2 2011-05-04 22:09:13

bobbym

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### Re: Cool way to square multiples of five

Hi 123ronnie321;

If a is the number you want to square and it ends in 5 then adding 5 and subtracting 5 will produce ( a + 5 ) ( a - 5).

So a^2 = ( a + 5 ) ( a - 5).= a^2 - 25

The only way to make this equation true is to add 25 to the RHS,

In mathematics, you don't understand things. You just get used to them.
I have the result, but I do not yet know how to get it.
All physicists, and a good many quite respectable mathematicians are contemptuous about proof.

123ronnie321
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Hi Bobbym,

## #4 2011-05-04 22:13:13

bobbym

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### Re: Cool way to square multiples of five

Hi;

How are you doing with the Vilenkin book?

In mathematics, you don't understand things. You just get used to them.
I have the result, but I do not yet know how to get it.
All physicists, and a good many quite respectable mathematicians are contemptuous about proof.

## #5 2011-05-04 22:22:04

123ronnie321
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### Re: Cool way to square multiples of five

I read the first few pages and found it interesting. The author has given some nice practical examples about where this field is used.
I have no idea about computers but he has given some nice information on "small" computer 'Strela'.

## #6 2011-05-05 01:46:31

bobbym

Online

### Re: Cool way to square multiples of five

Hi 123ronnie321;

Good! Combinatorics is a fun part of math. It is growing rapidly into one of the most important parts of math.

In mathematics, you don't understand things. You just get used to them.
I have the result, but I do not yet know how to get it.
All physicists, and a good many quite respectable mathematicians are contemptuous about proof.