title: is this expression O(n^2) or O(n^3)?
body: <p>Sum[(i + 1) (n - i), {i, 0, n - 1}]</p>
  
  <p>that is a sum of ( i+1)(n-1) with bounds from i=0 to n-1. </p>
  
  <p>is that O(n^2) or O(n^3)?
  and can you explain me how you found it? thanks.</p>