MHB Induction for divisibility by 10

AI Thread Summary
The discussion focuses on proving that for every n∈N, the expression 34n+2 + 1 is divisible by 10 using mathematical induction. The base case is established with n = 1, confirming that the expression holds true. The inductive hypothesis assumes that the expression is divisible by 10 for n = k, and the proof then shows it is also true for n = k + 1. The participants derive that both components of the expression maintain divisibility by 10 through their calculations. The proof is concluded by demonstrating that the inductive step successfully leads to the conclusion that the original statement holds for all natural numbers n.
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Show that for every n∈N, 34n+2 +1 is divisible by 10

Prove by Induction.

Attempt)

Base Case: n = 1, 3(4(1)+2) + 1 = 730
So the base case holds true.

Assume that the inequality holds for n = k

34k+2 +1 is divisible by 10

Show true for n = k+1

34(k+1)+2 + 1
34k+4+2 + 1
34 * 34k+2 + 1
81 * 34k+2 + 1
(80+1) * 34k+2 + 1
80(34k+2) + 34k+2 + 1

80(34k+2) + 34k+2 + 1 is divisible by 10 according to our induction hypotheses.What next?
 
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KOO said:
Show that for every n∈N, 34n+2 +1 is divisible by 10

Prove by Induction.

Attempt)

Base Case: n = 1, 3(4(1)+2) + 1 = 730
So the base case holds true.

Assume that the inequality holds for n = k

34k+2 +1 is divisible by 10

Show true for n = k+1

34(k+1)+2 + 1
34k+4+2 + 1
34 * 34k+2 + 1
81 * 34k+2 + 1
(80+1) * 34k+2 + 1
80(34k+2) + 34k+2 + 1

80(34k+2) + 34k+2 + 1 is divisible by 10 according to our induction hypotheses.What next?

$$3^{4k+2}=3^{2(2k+1)}=9^{2k+1}=(10-1)^{2k+1}$$
Regards,
$$|\pi\rangle$$
 
Hi KOO! :)

Let me continue from one of your expressions:
\begin{aligned}
81 \cdot 3^{4k+2} +1
&= 81 \cdot \left(3^{4k+2} + 1\right) - 81 + 1 \\
&= 81 \cdot \left(3^{4k+2} + 1\right) - 80
\end{aligned}

Both $\left(3^{4k+2} + 1\right)$ and $80$ are divisible by $10$...
 
Re: Induction:

I would make my induction hypothesis $P_k$ be:

$$3^{4k+2}+1=10m_k$$ where $$m_k\in\mathbb{N}$$

Next, as my inductive step, I would look at:

$$\left(3^{4(k+1)+2}+1 \right)-\left(3^{4k+2}+1 \right)=3^{4k+2}\left(3^4-1 \right)=80\cdot3^{4k+2}$$

Adding this to $P_k$, we find:

$$3^{4(k+1)+2}+1=10m_k+80\cdot3^{4k+2}=10\left(m_k+8\cdot3^{4k+2} \right)$$

If we use the recursive definition:

$$m_{k+1}=m_k+8\cdot3^{4k+2}$$ where $$m_1=730$$

then we may write:

$$3^{4(k+1)+2}+1=10m_{k+1}$$

Thus, we have derived $P_{k+1}$ from $P_k$ thereby completing the proof by induction.
 

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