What is the sum of the geometric sequence –3, 21, –147, … if there are 6 terms?

a. 11,772
b. –352,947
c. 44,118
d. –179,132

I think its c, am i right?

Question

What is the sum of the geometric sequence –3, 21, –147, … if there are 6 terms? 

a. 11,772
b. –352,947
c. 44,118
d. –179,132

I think its c, am i right?

zepp · Answer

Let's see.. this geometric series has a common ratio of 21/-3 = -7
First term is -3 and we are looking for the sum of the 6 first terms

$$\huge \sum_{k=0}^{6-1}ar^k=-3(\frac{1-(-7)^6}{1-(-7)})$$

zepp · Answer

It gave me -44118.75 :|

zepp · Answer

Oh, yeah, C is right, http://puu.sh/u5MN I just mistyped my numbers in my calculator ;D

anonymous · Answer

oh okay, thank u!