Find S20 for the series -1 + -3 + -5 + -7 +…

Question

Ultrilliam · Answer

@Vocaloid

sillybilly123 · Answer

I have a feeling that $S_{20}$ means the series [ie SUM] of the first 20 numbers in the sequence.

and not the 20th term of the sequence.

For the **given 4 terms**:

$S_4 =  -1 + -3 + -5 + -7$

We can also say, reversing the order, that:

$S_4 =  -7 + -5 + -3 + -1$

And then add them:

$2 \times S_4 =  -7 - 1 + -5 - 3 + -3 - 5 + -1 - 7  = -8 + -8 + - 8 + -8 = 4( - 8)$

it's -16 all the way but they want $S_{20}$

so you work that out to $S_{20}$, or generalise to $S_n$

...or you go look up a formula on Wiki for **arithmetic series**.

either gets you the answer :))