Home




Sequence and Series

Categories : Sequence and Series

GMAT Sequence and Series Arithmetic ProgressionThe sum of the first 6 terms of a geometric sequence is 9 times the sum of its first 3 terms. Find the common ratio.

A)    3
B)    4
C)    2
D)    1
E)    7


Solution

Sum of n terms of a geometric series is given by a ( r^n – 1) / r – 1    where a, r and n are the first term, ratio and number of terms of the series respectively.

Plugging in values, we get,

=> (a (r ^ 6 – 1) / r – 1) / (a (r ^ 3 – 1) / r – 1) = 9
=> (r ^ 6 – 1) / (r ^ 3 – 1) = 9

Assuming r ^ 3 as x, we get

=>  (x ^ 2 – 1) / (x – 1) = 9
=>  (x ^ 2 – 1) = 9 (x – 1)
=>  x ^ 2 – 9x + 8  = 0
=>  x = 8

Thus r ^ 3 = 8, and r = 2

Common ratio of the series is 2

Correct Answer: C

About the Author

...




Navigate F1GMAT


F1GMAT Services (MBA Applicants) 


MBA Research

MBA Salary(Latest Salary Data)
 
MBA Admission Interview Tips

Funding 

Deadlines

MBA Application Essays

GMAT Tutorials


GMAT Question Bank

Top MBA Programs


Get F1GMAT's Newsletters (Best in the Industry)
Included in the Newsletter:

  • Ranking Analysis
  • Post-MBA Salary Trends
  • Post-MBA Job Function & Industry Analysis
  • Post-MBA City Review
  • MBA Application Essay Tips
  • School Specific Essay Tips
  • GMAT Preparation Tips
  • MBA Admission Interview Tips
  • School Specific Interview Tips
  • Funding Guidance and
  • Special Consultation Service (only for Subscribers)

Subscribe to F1GMAT's Newsletter