Question

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

Answer 1

Do u mean the sum of the 15th term?

Answer 2

i wish. Lol i copied and pasted the problem. but i believe i need to find the 15th term?? maybe

Answer 3

No. I think it's finding the sum of that all the way to the 15th term

Answer 4

hmmmm its so confusing

Answer 5

So look at sum of arithmetic sequence
http://www.mathsisfun.com/algebra/sequences-sums-arithmetic.html

I know it's arithmetic because in order to get to the next u -2

Answer 6

Find the sum of the infinite geometric series 8 + 4 + 2 + 1 +...if it exists
that's another question on the test and it says sum.... but idk.

Answer 7

so would it be 52.5?

Answer 8

So for that formula
n=15
a=-1
d=-2

Answer 9

I was watching a youtube video from kahn academy and they said something about taking first term times last divided by two then times it by the term that youre wanting or something in this case 15?

Answer 10

Yes. If u don't remember it's in the link above

Answer 11

when i did that i got 52.5 did i do that right?

Answer 12

No. Should be -225

Answer 13

D: ahhhhh

Answer 14

15 [2(-1)+(15-1)(-2]
---
2

15[-2+14(-2)]
--
2

15(-30)
---------
2

=-225

Answer 15

where did you get the 2 at the very beginning? 15 [2(-1)+(15-1)(-2]

Answer 16

Part of the formula

n [2a+(n-1)d]
--
2

Answer 17

oh, i see. I believe im understanding it a lot more

Answer 18

Ok. Do u have any more questions?

Answer 19

not at the moment. Once i get to the end of the assignment im working on i think i might have a question

Answer 20

K