Which of these is the formula for the sum of the first nnn terms, SnS_nSn?
Sn=n2(a1+an)S_n = \frac{n}{2}(a_1 + a_n)Sn=2n(a1+an)
Sn=n(a1+an)S_n = n(a_1 + a_n)Sn=n(a1+an)
Sn=n2(a1−an)S_n = \frac{n}{2}(a_1 - a_n)Sn=2n(a1−an)
Sn=a1+(n−1)dS_n = a_1 + (n-1)dSn=a1+(n−1)d