n^3+(n+3)^3+(n+2)^3 is divisible by 9; n = N ; induction

To prove by induction that \( n^3 + (n+3)^3 + (n+2)^3 \) is divisible by 9 for all integers \( n \), let's follow the steps of mathematical induction.

### Step 1: Base Case

We start by checking the base case, where \( n = 0 \).

\[
n^3 + (n+3)^3 + (n+2)^3 = 0^3 + 3^3 + 2^3 = 0 + 27 + 8 = 35
\]

But 35 is not divisible by 9, which suggests a misinterpretation of the problem or a typo in the problem statement. 

Instead, let's try \( n = 1 \):

\[
n^3 + (n+3)^3 + (n+2)^3 = 1^3 + 4^3 + 3^3 = 1 + 64 + 27 = 92
\]

92 is not divisible by 9 either. It seems that \( n^3 + (n+3)^3 + (n+2)^3 \) may not be divisible by 9 for all \( n \). Let's recheck the statement and the base case.

However, if the statement were to be \( n^3 + (n+1)^3 + (n+2)^3 \) (as typically problems of this sort would involve consecutive integers), then we should analyze it further.

### Step 2: Let's Correct and Check for \( n^3 + (n+1)^3 + (n+2)^3 \)

Let's check if \( n^3 + (n+1)^3 + (n+2)^3 \) is divisible by 9.

#### Base Case for \( n = 0 \):

\[
n^3 + (n+1)^3 + (n+2)^3 = 0^3 + 1^3 + 2^3 = 0 + 1 + 8 = 9
\]

9 is divisible by 9, so the base case holds.

### Step 3: Inductive Step

Assume that for some integer \( k \), \( k^3 + (k+1)^3 + (k+2)^3 \) is divisible by 9.

So, assume:

\[
k^3 + (k+1)^3 + (k+2)^3 = 9m \quad \text{for some integer } m
\]

We need to prove that:

\[
(k+1)^3 + (k+2)^3 + (k+3)^3
\]

is also divisible by 9.

Expand the expression:

\[
(k+1)^3 + (k+2)^3 + (k+3)^3 = (k+1)^3 + (k+2)^3 + (k+3)^3
\]

Using the binomial theorem, we expand:

\[
(k+1)^3 = k^3 + 3k^2 + 3k + 1
\]
\[
(k+2)^3 = k^3 + 6k^2 + 12k + 8
\]
\[
(k+3)^3 = k^3 + 9k^2 + 27k + 27
\]

Summing these:

\[
3k^3 + 18k^2 + 42k + 36
\]

Factor out the 9:

\[
9(k^2 + 5k + 4) 
\]

Hence, the expression is divisible by 9. Therefore, the inductive step holds, and by the principle of mathematical induction, the original statement is true for all integers \( n \).

### Conclusion
The corrected statement \( n^3 + (n+1)^3 + (n+2)^3 \) is divisible by 9 for all \( n \), as shown by the inductive proof.

Would you like further details on this proof or any specific questions? Here are some related questions:

1. How does the binomial theorem work in the expansion of cubic expressions?
2. Can you show the expansion of another cubic term using binomial expansion?
3. What are common pitfalls when working with induction proofs?
4. How can we verify the correctness of an inductive hypothesis?
5. Why might the initial problem statement have led to a contradiction in the base case?

**Tip:** When using induction, always verify the base case thoroughly to ensure there are no initial contradictions.

n^3 + (n+3)^3 + (n+2)^3 is divisible by 9; n = N ; induction

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