Friday 24 March 2017

elementary number theory - Prove that $7^n+2$ is divisible by $3$ for all $n ∈ mathbb{N}$




Use mathematical induction to prove that $7^{n} +2$
is divisible by $3$ for all $n ∈ \mathbb{N}$.




I've tried to do it as follow.




If $n = 1$ then $9/3 = 3$.
Assume it is true when $n = p$. Therefore $7^{p} +2= 3k $ where $k ∈ \mathbb{N} $. Consider now $n=p+1$. Then
\begin{align}
&7^{p+1} +2=\\
&7^p\cdot7+ 2=\\
\end{align}

I reached a dead end from here. If someone could help me in the direction of the next step it would be really helpful. Thanks in advance.


Answer



Hint




If $7^n+2=3k$ then $$7^{n+1}+2=7(\color{red}{7^n})+2=7(\color{red}{3k-2})+2.$$


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