## segunda-feira, 4 de agosto de 2014

### Only for "geniuses" and not for stack exchangers

I came up with this quora question the other day about mathematicians and how they, along with most educated people, have some kind of difficulty on communicating with lesser educated ones (a.k.a. the public).

And most feedback I had there was not much different from what I had on a kind of similar post I've created months ago on stackexchange - educated people were putting me down, even while I got the sensation most of them didn't really get my question at all.

Now, it's no big news to anyone who read this blog (a.k.a. myself and my mom) that I suck at making myself understandable. I mean, it really isn't your fault if you can't understand whatever I say or write. I do believe I write enough you should be able to understand, but I often get myself with too many ambiguous messages when I re-read what I wrote, after understanding my dear readers point of view.

In any case, I think I also got to an answer to satisfy myself for that quora question: people are intellectually lazy. I'll write more about that later on, but it has a lot to do with definition of intelligence...

For now, I just want to start making a copy of all that. Because I like this subject and you never know when stackexchange community, quora or the internet will shut you down. So, redundancy copies! Whatever I write in my blog is automagically saved in my computer. ;-)

So, this is the question and answer I had on SEN before it got deleted, and how I can still see it right now:

# “Only for geniuses: if 2 = 6, 3 = 12, etc, then 9 equals?” [closed]

Last format I've seem of this was from this image: What's the correct answer, and why?
-

## closed as primarily opinion-based by Calvin Lin, Tomás, Clement C., Samrat Mukhopadhyay, Davide GiraudoFeb 26 at 13:40

Many good questions generate some degree of opinion based on expert experience, but answers to this question will tend to be almost entirely based on opinions, rather than facts, references, or specific expertise.If this question can be reworded to fit the rules in the help center, please edit your question or leave a comment.

## deleted by Community♦Mar 9 at 3:00

Considering that OP chose to answer this and claim that there is only 1 correct answer, I'm voting to close as this is "primarily opinion based". –  Calvin Lin Feb 26 at 13:08
@calvin i don't see what's wrong here. Can you please help me out? –  Cawas Feb 26 at 17:07

The question is very ill formatted in every aspect and the answer could be anything you want.
Among common answers people give to this question, maybe the simplest one is 90, and at least 2 ways to achieve that:
n * (n + 1) = S
2 * (2 + 1) = 6
3 * (3 + 1) = 12
...
6 * (6 + 1) = 42
9 * (9 + 1) = 90

Or with algorithms, which is simpler to think about than to write...
n = 2; S = 6;
loop:
print;
n = S + (6 + (n - 3) * 2);

which, in each line, will bring n and S values of (formula in middle):
n   formula                  S   comment
2                            6   (starting condition)
3 =  6 + (6 + (3 - 3) * 2) = 12  ( 6 +  6)
4 = 12 + (6 + (4 - 3) * 2) = 20  (12 +  8)
5 = 20 + (6 + (5 - 3) * 2) = 30  (20 + 10)
6 = 30 + (6 + (6 - 3) * 2) = 42  (30 + 12)
...                              (42 + 14, 56 + 16)
9 = 72 + (6 + (9 - 3) * 2) = 90  (72 + 18)

To get to 9, we ignore 7 and 8 results, but not the maths / algorithm behind it.
There are two other common answers: 56 and 72. You just ignore the left column skipping from 6 to 9. It's probably easier to understand seeing the numbers:
solution 56 - completely ignore left column
1:   2 * (2 + 1) = 6
2:   3 * (3 + 1) = 12
...
5:   6 * (6 + 1) = 42
6:   7 * (7 + 1) = 56

And then:
solution 72 - ignore left column only inside (n + 1)
1:   2 * (2 + 1) = 6
2:   3 * (3 + 1) = 12
...
5:   6 * (6 + 1) = 42
6:   9 * (7 + 1) = 72

-
I think you're wrong, it should be 0oeis.org/A077252 –  Daniel R Feb 26 at 13:12
This answer does not explain the use of the equals sign in the question - it gives formulae which compute to the numbers of the right hand side, but it doesn't explain why those formulae are equal to what appears on the left hand side. –  Mark Bennet Feb 26 at 13:28
If there is only one correct answer, then I think it should be: "anything you want". –  Etienne Feb 26 at 13:53
What did I do or say that was so wrong, guys? Daniel seem to be joking. @markbennet The equal sign is just wrong, but it makes no difference for the logic. But what you mean about left side formula? –  Cawas Feb 26 at 22:13
You didn't say or do anything "wrong". Apparently, many people just don't like this kind of questions. I agree with them (even though I didn't downvote). It just makes no sense to say that there is only one "correct answer". It is pretty easy to produce infinitely many simple algebraic formulas f(n) that will give the values 6,12,20,30,42 for n=2,3,4,5,6 and ... anything you want for n=7. –  Etienne Feb 26 at 22:59
Thanks @etienne. Do you think I should delete all this? –  Cawas Feb 27 at 3:34
Well, I really don't know. It's up to you. –  Etienne Feb 27 at 7:53