I've been working on a problem set for a bit now and I seem to have gotten the master method down for recurrence examples. However, I find myself having difficulties with other methods (recurrence trees, substitution). here is the question I am stuck on: $$T(n) = T(n-2) + n^2$$ Is there a pattern as follows? $$n^2 + T(n-2) + T(n-4) +...$$ where it goes until there is no more n left. so around n/2 times and would that mean that $$n^2 + (n-2)^2 + (n-i) ^2$$ so the asymptotic bound would be $\theta(n^2)$?
I am honestly taking a shot in the dark here, so I was hoping someone could help guide me in how to approach these questions.