I was reading the first chapter of Courant's and Robbins's "What is Mathematics", and there's a section on mathematical induction in which the authors go over the proofs of the arithmetic/geometric series formula (as they're proved inductively).
Recognizing that these proofs are more of a verification though (in page 15 it's mentioned that in some cases a "proof gives no indication of how this formula was arrived in the first place"), they show how you can derive these by playing around with the sums and their terms. For example, the arithmetic series formula for the first n integers can be found like this, but there's no mention as to why would anyone try to sum up these "sums" (sorry for being repetitive) with their terms rearranged that way. Same for the geometric series one (see here), where they're subtracted.
In summary, my question is: is there any other explanation to the discovery of these formulas? I don't think it was sheer luck. Who's credited for their creation? Is there any way to know their origin?
Thanks in advance for the help 🙂
Best Answer
Although others suggest this dates back to Euclid papers (which I would let for another answer if someone has more information about that), there are several sources suggesting that arithmetic and geometric series and possibly also some formulas/procedures to sum them up were already known to Egyptians sometime around $1650$ BC. Although it is certainly considered a speculation to a degree, I thought it is worth mentioning (and it is too short for a comment).
According to Expansions and Asymptotics for Statistics (Chapman & Hall/CRC Monographs on Statistics & Applied Probability, Page 1:
For the geometric series, there is for example a following problem according to Mathematics in Ancient Egypt: A Contextual History, Page $79$-$80$:
However footnote at the same page adds:
So we probably will never know for sure, but at least we know that Egyptians played with something what we nowadays call arithmetic and geometric series.