title: Solving a Recurrence Relation: T(n) = T(n-1) + n-1
body: <p>I have been asked to solve that recurrence relation. I got the next solution: <a href="https://imgur.com/a/xWoTI40" rel="nofollow noreferrer">https://imgur.com/a/xWoTI40</a></p>
  
  <p>So I decided to ask my teacher if it was right. He told me that it wasn't; and that this is the right solution: <a href="https://imgur.com/a/CGD0ta8" rel="nofollow noreferrer">https://imgur.com/a/CGD0ta8</a></p>
  
  <p>I'm totally clueless right now. The most I try to understand why mine is wrong; the most I think it's actually right.</p>
  
  <p>Can somebody explain?</p>