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#1 2011-05-03 23:45:06

123ronnie321
Member
Registered: 2010-09-28
Posts: 128

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?

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#2 2011-05-04 00:09:13

bobbym
Administrator
From: Bumpkinland
Registered: 2009-04-12
Posts: 83,072

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.

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#3 2011-05-04 00:11:35

123ronnie321
Member
Registered: 2010-09-28
Posts: 128

Re: Cool way to square multiples of five

Hi Bobbym,
up

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#4 2011-05-04 00:13:13

bobbym
Administrator
From: Bumpkinland
Registered: 2009-04-12
Posts: 83,072

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.

Online

#5 2011-05-04 00:22:04

123ronnie321
Member
Registered: 2010-09-28
Posts: 128

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'. smile

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#6 2011-05-04 03:46:31

bobbym
Administrator
From: Bumpkinland
Registered: 2009-04-12
Posts: 83,072

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.

Online

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