If `P(n)` is the statement `n^3+n` is divisible by 3, prove that `P(3)` is true but `P(4)` is not true.

Explanation

To solve the problem, we need to prove that the statement \( P(n) \) which asserts that \( n^3 + n \) is divisible by 3 is true for \( n = 3 \) and not true for \( n = 4 \).

### Step 1: Prove \( P(3) \)

1. Substitute \( n = 3 \) into the expression \( n^3 + n \):
\[ P(3) = 3^3 + 3 \]
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