I'm really sorry for this basic, stupid question. I have been looking for answers online but I can't find any.
I don't understand the following summation:
$$\sum_{i=0}^{n-1}i=\frac{n(n-1)}2$$
I understand that the basic formula for summations of i is the following:
$$\sum_{i=1}^ni=\frac{n(n+1)}2$$
I get how to do index shifts, but I don't see how we get to the result shown above. I understand that people value quality questions in this website; I did google around and found nothing, that is why I am here bothering you.
Thank you so much!