What is the sum of the geometric sequence –4, 24, –144, … if there are 6 terms?
a. –159,963
b. 26,660
c. 26,661
d. 159,963

I think its b, am i right?

Question

What is the sum of the geometric sequence –4, 24, –144, … if there are 6 terms?  
a. –159,963
b. 26,660
c. 26,661
d. 159,963

I think its b, am i right?

campbell_st · Answer

well you need the common ratio found by 

$$\frac{T _{2}}{T _{1}}=\frac{T _{3}}{T _{2}}...$$

as this is an alternating sequence the common ratio is negative

then use the formula

$$s _{6}=\frac{-4(r^6 -1)}{r-1}$$

anonymous · Answer

yea thanks i did that but im jsut checking to see if someone else got that answer as well.

campbell_st · Answer

use wolfram if you just want to check

anonymous · Answer

ok thanks~!