Hi, if there's anyone who could explain this to me, at least the first part, would be a great help! been going through some maths textbooks and none of them cover it.

Thanks!

(a) In an arithmetic progression, the 1st term is 13 and the 15th term is 111. Find

the common difference and the sum of the first 20 terms.

(b) Express the sum obtained in part (a) using the sigma notation.