Do we just cube everything?

Algebra Level 2

Fill in the blank:

$\begin{array} { l l l l l l l l l l l } 1 & + & 2 & + & 3 & + & \cdots & + & n & = &300 \\ 1^3 & + & 2^3 & + & 3^3 & + & \cdots & + & n^3 & = &\text{\_\_\_\_\_\_\_}. \end{array}$