What is the sum of an 8-term geometric series if the first term is 14 and the last term is -3,919,104?

Question

campbell_st · Answer

well you need to find the common ratio. 

to do this you need the information for the 8th term 

a term in a geometric series is 

$$a_{n} = a \times r^{n -1}$$

so you know 

$$-3919104 = 14 \times r ^7$$

solve for r. 

then the sum of a geometric series is 

$$S_{8} = \frac{14( 1 - r^8)}{1 - r}$$

hope this helps.

anonymous · Answer

−3919104=14×r^7
or r^7 = -279936
or r^7 = -(2*3)^7
hence r=-6  use as campbell_st has formulated

campbell_st · Answer

got to love the answer post... eliminates the need for learning.