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?

Answer 1

@aum

Answer 2

For a finite geometric sum, recall the formula \(\displaystyle \sum_{k=1}^{n} ar^{k-1} = a\dfrac{1-r^n}{1-r}\), where \(a\) is the first term, \(n\) is the number of terms in our series, and \(r\) is the common ratio.

In this case, you're given that a=-11, n=7, and r=-5. What do you get when you plug everything into that formula?

Can you take things from here? :-)

Answer 3

noo

Answer 4

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

Answer 5

@ChristopherToni

Answer 6

What do you get when you evaluate \(\displaystyle -11\cdot\frac{1-(-5)^7}{1-(-5)}\) with your calculator?

Answer 7

my calculator says error

Answer 8

-859441

Answer 9

@ChristopherToni