Find sum of first n terms of arithmetic series (#427)

Find sum of first n terms of arithmetic series (#427)

Use Sn=n2(a1+an)S_n = \frac{n}{2}(a_1 + a_n) or Sn=n2(2a1+(n−1)d)S_n = \frac{n}{2}(2a_1 + (n-1)d).