Question

Find S15 for the series -1 + -3 + -5 + -7 +...

Answer 1

The sum? or the 15th term in the sequence?

Answer 2

Sum of an arithmetic series:
\[S_n = \frac{n(a_1 + a_n)}{2}\]
where
\[a_n = a_1 + d(n-1)\]
\[a_1 = -1\]
\[d = -2\]