# Sequence of numbers

• September 27th 2011, 01:58 PM
seventhson
Sequence of numbers
I have a number theory problem with solution but i dont understand why one can write this equallity (green question mark). Because 1122 is not 11*22 or 111222 is not 111*222.
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• September 27th 2011, 02:22 PM
ebaines
Re: Sequence of numbers
The attachment has a typo. It should be:

$11..122...2 = \frac 1 9 (10^{2n} - 10^n) + \frac 2 9 (10^n - 1)$

The rest is correct. For example, starting with 1122 (n=2) you have:

$1122 = 1100 + 22 = \frac 1 9 9900 + \frac 2 9 99 = \frac 1 9 (10^4 - 10^2) + \frac 2 9 (10^2 -1)$
$= \frac 1 9 (10^4 +10^2 - 2) = \frac 1 9 (10^2-1)(10^2+2) = \frac 1 9 99 \cdot 102 = 33 \cdot 34$
• September 27th 2011, 11:32 PM
agentmulder
Re: Sequence of numbers
To me it looks like the expression immediately after the green question mark and before the next equal sign...

1/9(10^n -1).2/9(10^n -1)

i think that dot separates 1's and 2's

Examples: n = 1

1/9(9).2/9(9) = 1.2, '1 dot 2' or twelve

n = 2

1/9(99).2/9(99) = 11.22, '11 dot 22' or 1122

n = 3

1/9(999).2/9(999) = 111.222, '111 dot 222' or 111222

Am I stretching it too much?

(Thinking)
• September 28th 2011, 01:29 AM
seventhson
Re: Sequence of numbers
Thank you very much. You have nice brains.
• September 28th 2011, 06:40 AM
ebaines
Re: Sequence of numbers
Quote:

Originally Posted by agentmulder
i think that dot separates 1's and 2's
...
1/9(999).2/9(999) = 111.222, '111 dot 222' or 111222

Am I stretching it too much?

Nice idea - but yes, you are stretching it too much! By convention the "dot" symbol means multiplication. Thus:

$11 \cdot 22 = 11 \times 22 = 242$