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

-143231

Answer 2

sum = -11 * (-5)^7 - 1
---------
-5-1

Answer 3

@jimmyrep : \( 1 -(-5) \) isn't ?

Answer 4

r is less than 1

Answer 5

yea - u r right it should be 1 - r since r < 1

Answer 6

sum = -11 * (1 - (-5)^7)
--------
1 - (-5)

Answer 7

= -143231

Answer 8

thank you! so you do the fraction up top first then divide it by the bottom and then multiply right orr?

Answer 9

yea - you can do the calculation that way

you know something though- although its usual to use 1-r^n and 1-r if r < 1
i'm not sure that it matters - i think it will give same answer - i'll check that out with this question

Answer 10

yea - it gives same answer

Answer 11

okay thanks!

Answer 12

@jimmyrep: Try taking limit when \( n \to \infty \)

Answer 13

yes - i know what you are saying - its only that it works in a case like this one - however as a general rule you should use 1 - r when r < 1

Answer 14

:)