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Arithmetic summation question - I don't understand this answer.

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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!


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