Why are these summations not equivalent?

Hello i have this sum:

$\sum_{j=0}^{n-2}(n-j)$

i try solve of this mode:

$\sum_{j=0}^{n-2}n - \sum_{j=0}^{n-2}j$ or $\sum_{j=0}^{n-2}n + \sum_{j=0}^{n-2}-j$

but in wolfram alpha it not same, and the expected result is n²

This sum is obtained from the following recurrence:

$f(n)= 1 if n<= 1\\
f(n)= n+f(n-1) n>1$

but it not same