In an Arithmetic progression of n terms, the common differences is d and the last terms is l .how to find first two terms in d , l , n term?.....

Is it use formula S_n=n/2(a+l) or S_n=n/2(2a+(n-1)d)?..i m confusing abt the way to solve...can anybody help me?

Sent from my Lenovo A5000 using Tapatalk