Cool way to square multiples of five

123ronnie321 · 2011-05-03 23:45:06

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

Can you guess why it works?

bobbym · 2011-05-04 00:09:13

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,

123ronnie321 · 2011-05-04 00:11:35

Hi Bobbym,

bobbym · 2011-05-04 00:13:13

Hi;

How are you doing with the Vilenkin book?

123ronnie321 · 2011-05-04 00:22:04

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'.

bobbym · 2011-05-04 03:46:31

Hi 123ronnie321;

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

Math Is Fun Forum

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

Cool way to square multiples of five

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

Re: Cool way to square multiples of five

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

Re: Cool way to square multiples of five

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

Re: Cool way to square multiples of five

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

Re: Cool way to square multiples of five

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

Re: Cool way to square multiples of five

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