# Cube as Difference between Two Squares

## Theorem

A cube number can be expressed as the difference between two squares.

## Proof

 $\displaystyle n^3$ $=$ $\displaystyle \sum_{k \mathop = 1}^n k^3 - \sum_{k \mathop = 1}^{n - 1} k^3$ $\displaystyle$ $=$ $\displaystyle \paren {\frac {n^2 \paren {n + 1}^2} 4} - \paren {\frac {\paren {n - 1}^2 n^2} 4}$ Sum of Sequence of Cubes $\displaystyle$ $=$ $\displaystyle \frac {n^2 \paren {\paren {n + 1}^2 - \paren {n - 1}^2} } 4$ $\displaystyle$ $=$ $\displaystyle \paren {\frac {n \paren {n + 1} } 2}^2 - \paren {\frac {n \paren {n - 1} } 2}^2$

$\blacksquare$