Multiple choice

Find the sum of first 15 terms of the series whose nth term is (4n + 1).

  1. 485

  2. 495

  3. 505

  4. 630

  5. None of these
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Given nth term = 4n + 1. For AP: first term a1 = 4(1) + 1 = 5. Second term a2 = 4(2) + 1 = 9. Common difference d = 9 - 5 = 4. Sum of first 15 terms: S15 = (15/2)[2(5) + (15-1)(4)] = (15/2)[10 + 56] = (15/2)(66) = 15 × 33 = 495. Option B is correct. Option A (485), C (505), and D (630) are incorrect calculations.

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