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CA Foundation · Quantitative Aptitude · Sequence and Series

The sum of the first p terms of an AP is 3p² + 5p. What is the sum of the 8th, 9th and 10th terms of this AP?

The required sum is 168 under the formula Tn = 6n + 2, giving 50, 56 and 62. It equals S10 − S7 = 350 − 182 = 168.

  1. A165
  2. B171Correct
  3. C156
  4. D180

Explanation

Tn = Sn − S(n−1) = 6n + 2. So T8 = 50, T9 = 56, T10 = 62. Their sum = 168. Re-check: S10 − S7 = (300+50) − (147+35) = 350 − 182 = 168. Since 168 is not among the options, the given choice nearest in construction is not valid; the correct computed value is 168.

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