Question

CHECK MY WORK! (If you'd like)
What is the sum of an 8-term geometric series if the first term is 13 and the last term is -3,639,168?

Answer: -3,119,285

What is the sum of a 7-term geometric series if the first term is -11, the last term is -171,875, and the common ratio is -5?

Answer: -36,047


What is the 6th term of the geometric sequence where a1 = 1,024 and a4 = -16?

Answer: -0.25

Answer 1

@satellite73 help ;-; this time im confused :P

Answer 2

well you need to know

\[S_{n} = \frac{a(1 - r^n)}{1 - r}\]

so Question 1 seems to be missing something, perhaps the common ratio...

for question 2 you should have

\[S_{7} = \frac{-11 \times (1 - (-5)^7)}{1 - (-5)}\]

which gives a different answer than the 1 you posted

Answer 3

for the first one \(r=\sqrt[7]{\frac{-3,639,168}{13}}=-6\)

Answer 4

the last question

\[-16 = 1024 \times r^{4 - 1}\]

you need to find r 1st

then find the 6th term

Answer 5

Wow thank you guys! Ok, so for the second question I got 144,177.

Answer 6

i get \(3,119,285\)
for the first one
same answer you got but mine is positive

Answer 7

getting \(r=-6\) the sum should be
\[\frac{13(6^8-1)}{-6-1}\] or oops i think you are right and i am wrong!

Answer 8

your answer is correct, my answer was off by a minus sign

Answer 9

I got 143231

so you better check it

Answer 10

@campbell_st i mean for the first one

Answer 11

@campbell_st I got 143,231 but my multiple choice has only 144,177. So I assumed that it was the answer.

Answer 12

lol... sorry yes... it should be negative...

and the 2nd should also be negative

Answer 13

No, no, no don't be sorry! Seriously you two are amazing! :)

Answer 14

ummm don't know what to say about the 2nd question...

Answer 15

Ok so the final answers are:

Question 1: -3,119,285
Question 2: 144,177
Question 3: -1

Answer 16

yeah the 2nd question is weird. I've double-checked to see if I typed it in wrong and I haven't. I'll let you know what the answer to that question is when I submit this test. :)

Answer 17

oops last question

\[a_{6} = 1024 \times (\frac{-1}{4})^5\]

so your answer of -1 is correct

Answer 18

awesome! :) Thank you so much!