Question

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

Answer 1

Do you need the sum?

Answer 2

Yes

Answer 3

The formula for the sum of n terms of an arithmetic progression is
\[S=\frac{n}{2}[2a+(n-1)d]\]
where a is the first term and d is the common difference.
Plugging the given values into the formula gives
\[S=\frac{15}{2}[-2+(15-1)\times -2]=\frac{15}{2}\times -30\]