I am having a little trouble following an example I came across today which says that:
$2 \sum_{k=1}^{n} \sum_{i=0}^{k-2} 1 = 2 \sum_{k=1}^{n} (k-1) = 2 \sum_{j=0}^{n-1} j$
I have tried fidgeting around with the expressions, but I just don't follow the thought process here. If anyone can please help me with some intermediate steps here, I will be very grateful!