The problem states that if $n$ is an odd integer, then $n^3$ is odd. We need to prove this statement.

Number TheoryInteger PropertiesOdd and Even NumbersProof by direct method
2025/6/7

1. Problem Description

The problem states that if nn is an odd integer, then n3n^3 is odd. We need to prove this statement.

2. Solution Steps

If nn is an odd integer, then it can be represented as:
n=2k+1n = 2k + 1, where kk is an integer.
Now we want to find n3n^3:
n3=(2k+1)3n^3 = (2k + 1)^3
n3=(2k+1)(2k+1)(2k+1)n^3 = (2k + 1)(2k + 1)(2k + 1)
n3=(4k2+4k+1)(2k+1)n^3 = (4k^2 + 4k + 1)(2k + 1)
n3=8k3+4k2+8k2+4k+2k+1n^3 = 8k^3 + 4k^2 + 8k^2 + 4k + 2k + 1
n3=8k3+12k2+6k+1n^3 = 8k^3 + 12k^2 + 6k + 1
We can rewrite this as:
n3=2(4k3+6k2+3k)+1n^3 = 2(4k^3 + 6k^2 + 3k) + 1
Let m=4k3+6k2+3km = 4k^3 + 6k^2 + 3k, where mm is an integer.
Then n3=2m+1n^3 = 2m + 1
Since n3n^3 can be written in the form 2m+12m + 1, where mm is an integer, n3n^3 is an odd integer.

3. Final Answer

If nn is an odd integer, then n3n^3 is odd.

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