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## #1 Puzzles and Games » Divisibility - neat little puzzle » 2008-07-10 05:15:05

Maracuya
Replies: 2

I came accross this on yahoo answers. :

Think your'e good at math? Try this one!?
There is a 9-digit number containing the numbers 1 through 9. Each number is only used once. This 9-digit number is divisible by 9. When you take off the right-most digit, it become an 8-digit number that is divisible by 8! When you take off the last digit to create a 7-digit number, the whole number is now divisible by 7. The pattern continues all the way down until the end, when it is a two digit number divisible by two, and then taking off the end digit creates a 1-digit number, divisible by 1.
HINTS:
*The numbers are not in order.
*Since 1+2+3+4+5+6+7+8+9=45 and 45 is divisible by 9, that clue does not help us. No matter what order the numbers are it will be divisible by 9 in the begining.
*Think about the 5th digit... what would make the number divisible by 5 when it is only 5 numbers long? There's one filled in for you!
*Review your divisibility rules.... try google to get them all. *Have fun!

The original post:

## #2 Re: Maths Is Fun - Suggestions and Comments » Logging out » 2008-06-25 18:01:34

I'd like to suggest you bump this up to a higher priority.
Reason number two affects me personally, but numbers 1 and 3 should probably be considered.
1)  It is potentially a security issue for kids who use this site at school (it seems the login never times-out.)
2)  I decided I would just stay logged in all the time.  But to see a list of new threads, I need to mess with the cookies, which is getting annoying.
3) This is a public relations issue -- it detracts from the overall classiness and usability of the site.

## #3 Re: Maths Is Fun - Suggestions and Comments » Worked Exercises "Repository" » 2008-06-15 23:05:05

I'd be happy to contribute some exercises, but am too new to math to be a "checker".
Can I post problems I've seen online, or from texts I've used?  If so, should I give credit to the author/s?

If you want only original problems, I'd enjoy writing some.
Is this project going forward? What has been done, and what needs to be done?

As a footnote, I've just started working through Spivak's Calculus.  Some of the problems are so good that I've been tempted to post them for other members to enjoy and discuss.
In general, I'm wondering how much material from a single book can be posted, without being illegal/ unethical.

Mara

## #4 Re: Maths Is Fun - Suggestions and Comments » Infinity » 2008-06-15 19:50:10

I liked your explanation of "Isn't ∞ / ∞ equal to 1?"

I was not  as clear on why  0 × ∞ , ∞^0 , and ∞ - ∞ are undefined.

Questions/ clarifications:
1) Is ∞ = ∞ defined?  I'm guessing that it is not,  since ∞ -∞  is not defined.
2)You mentioned different magnitudes of infinity.  Is the symbol ∞ only for the smallest (Aleph-null ) infinity, or is it generic for infinity of any size?
3) n^0 is defined to be equal to 1 for any n not equal to zero, because this makes the operations consistent for numbers raised to any power.
Is there a similar logic to why ∞^0 must not equal one?

## #5 Re: Maths Is Fun - Suggestions and Comments » Logging out » 2008-06-04 21:13:18

Today, I was unable to logout until I deleted my cookies.  I don't recall  having this trouble before.
No big deal, just a heads-up for MathIsFun.

(BTW, thanks for the great forum !)

## #7 Re: Help Me ! » Nested absolute value in algebraic expression » 2008-06-01 17:54:57

To meo_beo , Ricky, aleclarsen12 thanks  for the replies.

To :aleclarsen12 , I'm sure Ricky gave you good advice.  But...

here's a problem for you where your definition will make a problem much simpler, while the piecewise definition that I gave will make your life hard!

Try to prove this fact :

This is from a college level book, so tell me if you want more hints.

Maracuya

## #8 Help Me ! » Nested absolute value in algebraic expression » 2008-06-01 15:09:18

Maracuya
Replies: 6

I need to express this with "at least one less pair of absolute value signs."

Since

,  the expression inside the parenthesis must be negative or zero.

So have I got it, or am I missing something important.?

Thanks

## #9 Re: Help Me ! » LaTeX - A Crash Course » 2008-05-24 20:20:00

Today I started learning Latex on Texmaker (for Linux).  The same code that produces this nice looking limit on this website (from Dross' tutorial) comes out badly in my dvi files.

I want it to look like this:

is to the right of lim , rather than directly under.

Why this variation from system to system?  How can I get mine to look correct in my documents?

edit: A partial answer to my own question is to use the \displaystyle command.  I still don't completely understand this command.

Conversely, if I want to be sure that the limit is displayed to the right, rather than under, I can use the \nolimit  command

## #10 Help Me ! » definition of percentile? » 2008-02-04 13:41:26

Maracuya
Replies: 2

I have read various web definitions of percentile, and there is some disagreement.  In you answers,  please explain in a way that I can understand without having taken a statistics course.

So what is you definition, and what would you consider an authoritative web source for the answer?

I will post a follow-up with the context for my question, which is a programming assignment for an intro to data structures class.