When dealing with a finite arithmetic series in summation notations and the index of summation (i) is greater than 1, would you plug in 1 or that index of summation number to find the first term?

Question

Let’s say I am trying to find n (the upper limit of summation) and I am given the index of summation is i=3...the equation I am given next to the summation notation is (-3-4i) and the sum is -507. I am confused how you would set this up would you plug in 3 into (-3-4i) to find the first term and then when you set it up in the equation it would be [(n-2) (-15 + -3-4n)/2]? Or do you always plug in 1 to find the first term, regardless of the index of summation, so that it would be [(n) (-7 + -3-4n)/2]? PLEASE HELP!!!!

Answer 1

You would plug in the number they give you for the lower limit to find the first term.