I am trying to find Big O of this formula: $T(n)=T(n-1)+2n$ by using iteration however I am stuck on a step.
$T(n)=T(n-1)+2n$ I then plugged $T(n-1)$ into the equation so $T(n-1)=T(n-1-1)+2(n-1)$ which then I got $T(n)=T(n-2)+2(n-1)+2n$. $T(n-2)$ gives $T(n-3)+2(n-3)$ which plugging it back in gives $T(n)=T(n-3)+2(n-3)+2(n-1)+2n$. I substituted the following $2(n-3)+2(n-1)+2n$ into a summation that gave me $\sum_{i=0}^k 2(n-i)$$=-(k+1)(k-2n)$. I am not too sure how I should continue.