The sum of the first n positive integers is .

So the first term is the sum of the first 50 positive integers is 



Then to get the second term, we add 1 to each one of those 50, which adds 50.
We add 50 each time to get the next term.

So it's an arithmetic series with 1st term 1275 and common difference 50,
with 50 terms.





That works out to be 125000.

Edwin