# Divisible by 49

Show 40 post(s) from this thread on one page
Page 1 of 2 12 Last
• Aug 23rd 2006, 06:45 PM
OReilly
Divisible by 49
Can someone show me the proof by mathematical induction that $\displaystyle n^2+n+2$ isn't divisible by 49 for every n (natural number).
• Aug 23rd 2006, 06:55 PM
ThePerfectHacker
Quote:

Originally Posted by OReilly
Can someone show me the proof by mathematical induction that $\displaystyle n^2+n+2$ isn't divisible by 49 for every n (natural number).

You can greatly simplify your problem to reducing it to showing that $\displaystyle 7\not |n^2+n+2$
• Aug 23rd 2006, 07:09 PM
OReilly
Quote:

Originally Posted by ThePerfectHacker
You can greatly simplify your problem to reducing it to showing that $\displaystyle 7\not |n^2+n+2$

But it can be divisible by 7. For example for n=3.
• Aug 23rd 2006, 07:23 PM
ThePerfectHacker
Quote:

Originally Posted by OReilly
But it can be divisible by 7. For example for n=3.

I know, but once you exceede 49 it is always true.

I was just writing out a proof. But I accidently closed my internet thing and lost everything I typed.
• Aug 23rd 2006, 10:32 PM
rgep
Firstly observe that n^2+n+2 is divisible by 7 if and only if n is congruent to 3 modulo 7: that is, if n is of the form 7x+3. See this by writing it as n^2 - 6n + 9 + 7(n-1), so that it is (n-3)^2 modulo 7.

Now substitute n=7x+3 into n^2 + n + 2 to get 49x^2 + 49x + 14, which cannot be divisible by 49.
• Aug 24th 2006, 06:23 AM
OReilly
Is this proof valid?

Supose that for $\displaystyle n=k$ expression is divisible by 49:
$\displaystyle k(k + 1) + 2 = 49a$

Supose that now for $\displaystyle n=k+1$ expression is also divisible by 49:
$\displaystyle (k + 1)(k + 2) + 2 = 49b$

From first equation we get that $\displaystyle k + 1 = \frac{{49a - 2}}{k}$.

Substituting into second equation we get:
$\displaystyle \begin{array}{l} (\frac{{49a - 2}}{k})(k + 2) + 2 = 49b \\ \frac{{49ak + 98a - 2k - 4}}{k} = 49b - 2 \\ 49ak + 98a - 2k - 4 = 49bk - 2k \\ 49ak + 98a - 2k - 4 - 49bk + 2k = 0 \\ 49ak + 98a - 49bk - 4 = 0 \\ \end{array}$

Last expression isn't divisible by 49 so $\displaystyle n^2+n+2$ isn't divisible by 49.
• Aug 24th 2006, 02:40 PM
ThePerfectHacker
You need to show that,
If,
$\displaystyle n^2+n+2\not = k$
then,
$\displaystyle n^2+n+2\not = k+1$

Equivalently (contropositive),
If,
$\displaystyle n^2+n+2=k+1$
then,
$\displaystyle n^2+n+2=k$

You shown that:
if,
$\displaystyle n^2+n+2=k$
then,
$\displaystyle n^2+n+2\not = k+1$
This is not equivalent to the above two statements.
• Aug 24th 2006, 05:10 PM
Quick
I don't get it...

where am I off?

If $\displaystyle n^2+n+2=k$
then, $\displaystyle n^2+n+3=k+1$

:confused:
• Aug 24th 2006, 05:57 PM
ThePerfectHacker
Here is how you do it.
---
You need to show there doth not exist such a $\displaystyle 49k$ such as,
$\displaystyle n^2+n+(2-49k)=0$
Which means there are no integral solution for $\displaystyle n$.
If you solve for "n",
$\displaystyle n=\frac{-1\pm \sqrt{1-4(2-49k)}}{2}$.
Note the following, if the expression in the radical is a square then "n" is an integral number because the square root of an odd square is an odd whole number and when added of subtracted to 1 produces an even number and when finally divided by two produces an integral number, thus, if
$\displaystyle 1-4(2-49k)=196k-7$ is not a square then,
$\displaystyle 1-4(2-49(k+1))=1-4(-47-49k)=196k+189$ is also not a square.
Thus, again you need to show that.
IF,
$\displaystyle 196k-7$ is not a square
THEN,
$\displaystyle 196k+188$ is not a square.
~~Contropositive Time~~
IF,
$\displaystyle 196k+188$ is a square.
THEN,
$\displaystyle 196k-7$ is a square.

For some reason I cannot continue. I am hoping someone will see this and tell me the next step.
• Aug 24th 2006, 09:15 PM
CaptainBlack
Quote:

Originally Posted by OReilly
Is this proof valid?

Supose that for $\displaystyle n=k$ expression is divisible by 49:
$\displaystyle k(k + 1) + 2 = 49a$

Supose that now for $\displaystyle n=k+1$ expression is also divisible by 49:
$\displaystyle (k + 1)(k + 2) + 2 = 49b$

From first equation we get that $\displaystyle k + 1 = \frac{{49a - 2}}{k}$.

Substituting into second equation we get:
$\displaystyle \begin{array}{l} (\frac{{49a - 2}}{k})(k + 2) + 2 = 49b \\ \frac{{49ak + 98a - 2k - 4}}{k} = 49b - 2 \\ 49ak + 98a - 2k - 4 = 49bk - 2k \\ 49ak + 98a - 2k - 4 - 49bk + 2k = 0 \\ 49ak + 98a - 49bk - 4 = 0 \\ \end{array}$

Last expression isn't divisible by 49 so $\displaystyle n^2+n+2$ isn't divisible by 49.

It looks to me that what you have proven is that $\displaystyle k^2+k+2$ is not divisible
by $\displaystyle 49$ for consecutive integral values of $\displaystyle k$.

RonL
• Aug 24th 2006, 09:35 PM
CaptainBlack
Quote:

Originally Posted by Quick

where am I off?

If $\displaystyle n^2+n+2=k$
then, $\displaystyle n^2+n+3=k+1$

:confused:

From his silence on this and departure off on another tangent I think
ImPerfectHacker agrees with you and he doesn't understand his
argument either :D

It is either so subtle that we can't see the point, or its wrong. I know
where I'm putting my money (no not in that box under the bed).

RonL
• Aug 25th 2006, 04:32 AM
ThePerfectHacker
Look at post #7. I say it is no good.
---
I still do not see why you chose such a method. The quickest would be quadradic reciprocity.
• Aug 25th 2006, 05:54 AM
Glaysher
Quote:

Originally Posted by ThePerfectHacker
You need to show that,
If,
$\displaystyle n^2+n+2\not = k$
then,
$\displaystyle n^2+n+2\not = k+1$

Equivalently (contropositive),
If,
$\displaystyle n^2+n+2=k+1$
then,
$\displaystyle n^2+n+2=k$

You shown that:
if,
$\displaystyle n^2+n+2=k$
then,
$\displaystyle n^2+n+2\not = k+1$
This is not equivalent to the above two statements.

Don't you mean

Need to prove either

statement true for n = k implies statement true for n = k + 1

or

Contropositive
Statememt not true for n = k + 1 implies statement not true for n = k

OReilly showed

Statement true for n = k implies statement not true for n = k + 1
• Aug 25th 2006, 06:34 AM
Quick
Quote:

Originally Posted by Glaysher
Don't you mean

Need to prove either

statement true for n = k implies statement true for n = k + 1

wouldn't it imply that n+1=k+1???
• Aug 25th 2006, 08:56 AM
Glaysher
Quote:

Originally Posted by Quick
wouldn't it imply that n+1=k+1???

I assume you are unfamiliar with the principle of mathematical induction

Usually you are trying to prove something for all natural numbers n

You show that it is true when n = 1

Then you assume it is true for some n = k

It is okay to assume this as we have already proved it for n = 1 so we already know there is at least one possible value for k.

We then try to prove that it also holds for n = k + 1 assuming that it holds for n= k.

Once we have done this we have proved it for all natural numbers n as

True for n = 1 means true for n = 2 means true for n = 3 and so on ...

This method can be adapted to suit several different purposes
Show 40 post(s) from this thread on one page
Page 1 of 2 12 Last