Question

In terms of n, find the sum of the first n terms of the arithmetic sequence: -4, 5, 14, 23, 32, .....

Answer 1

\[a _{n} = a _{1} + (n - 1)d\]
\[a _{n} = -4 + 9n - 9 = 9n - 13\]
\[S_{n} = n/2(a _{1} + a _{n})\]
\[S _{n} = n/2(-4 + 9n - 13)\]
\[n(9n - 17)\over2\]
\[9n ^{2} - 17n \over2\]