# Math Help - Not prime number

1. ## Not prime number

Prouve that number is not prime number :
(x is whole number no zero ) .

2. is that the entire question? i feel there is something missing here...

3. prove that this is not a prime.

if x=1

2 to the power of 2x +1 + 1 would equal

2 to the power 3 +1

8 + 1 = 9 which is not a prime

4. Hello, dhiab!

Prove that $N \;=\;2^{2n+1}+1$ is not a prime number, for any positive integer $n.$

$N \;=\;2^{\text{odd power}} + 1$ is always factorable.

. . $2^{2n+1} + 1 \;=\;(2 + 1)\left(2^{2n} - 2^{2n-1} + 2^{2n-2} + \hdots + (\text{-}1)^{n+1}\right)$

Therefore, $N$ is always a multiple of 3.

5. [quote=Soroban;326807]Hello, dhiab!

$N \;=\;2^{\text{odd power}} + 1$ is always factorable.

. . $2^{2n+1} + 1 \;=\;(2 + 1)\left(2^{2n} - 2^{2n-1} + 2^{2n-2} + \hdots + (\text{-}1)^{n+1}\right)$

Therefore, $N$ is always a multiple of 3.

[/quote
Thank you for this resolution ?This my resolution (Theorem of induction)
I prouve that : :
- for : I have .
-Now . I PROUVE FOR :

6. He3llo, again, dhiab!

This is my resolution (induction)

I prove that: . $\forall x \in I^+,\;2^{2x+1} + 1 \:=\:3k$

For $x = 1$, I have: . $2^{2(1)+1} + 1 \:=\:2^3+1 \:=\:9 \:=\:3\cdot3$

Now: . $2^{2x+1} + 1 \:=\:3k\:\text{ for some integer }k.$

Prove for $x+1$:

$2^{2(x+1)+1}+1 \;=\;2^{2x+3} + 1 \;=\;4\cdot2^{2x+1} + 1 \;=\;3\cdot2^{2x+1} + \underbrace{(2^{2x+1}+1)}_{\text{This is }3k}$
Therefore: . $2^{2(x+1)} + 1 \;=\;3\cdot2^{2k+1} + 3k \;=\;3\bigg[2^{2k+1} + k\bigg] \;=\;3m\:\text{ for some integer }m.$