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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
bumpkin
From: Bumpkinland
Registered: 2009-04-12
Posts: 109,606

### 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.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.

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

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

Hi Bobbym,

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

bobbym
bumpkin
From: Bumpkinland
Registered: 2009-04-12
Posts: 109,606

### 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.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.

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## #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'.

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

bobbym
bumpkin
From: Bumpkinland
Registered: 2009-04-12
Posts: 109,606

### 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.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.

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