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CA Foundation · Quantitative Aptitude · Sequence and Series

The sum of the first n terms of a series is given by Sn = 3n² + 2n for every n. What is the 10th term of the series?

The 10th term is 59. A term of a series equals the sum up to that term minus the sum up to the previous term. Here S10 is 320 and S9 is 261, so the difference is 59. Equivalently, the general term is 6n − 1.

  1. A59Correct
  2. B61
  3. C320
  4. D57

Explanation

The nth term is Sn − S(n−1) = 3n² + 2n − [3(n−1)² + 2(n−1)] = 6n − 1. For n = 10 this gives 59. Check: S10 = 320 and S9 = 261, so the difference is 59. The option 320 is just S10, not the 10th term.

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