Question

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?

-143,231
144,177
716,144
-36,047

Answer 1

@ParthKohli would you mind helping me through this?

Answer 2

@whpalmer4

Answer 3

I don't know where to start? Which formula?

Answer 4

\[Sn=a \frac{ r^n-1 }{ r-1 }\]
n=7
a=-11
r=-7
calculate S7

Answer 5

S7=-11 - 823544/ -8, and after calculating I get 102932 but this isn't an option :(

Answer 6

@surjithayer

Answer 7

Am I calculating wrong?

Answer 8

Hello?

Answer 9

sorry r=-5

Answer 10

I got 28645.8333. this isn't right. Can you please walk me through it?

Answer 11

then r is not =-5
let me calculate

Answer 12

following the formula given to u: a=-11 and r=-5 so
\[\large S_7=(-11)\frac{(-5)^7-1}{(-5)-1}=(-11)\frac{-78126}{-6}=-143231 \]

Answer 13

ahh, thank you <33333

Answer 14

u r welcome

Answer 15

so your calculation was wrong.