Multiple choice

Directions: Type in your answer in the input box provided below the question. Find the sum up to 30 terms of the series S = 2 + 6 + 12 + 20 + 30 +....

  1. 9900

  2. 9910

  3. 9920

  4. 9940
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

The series is 2, 6, 12, 20, 30... which is n(n+1). Sum = sum(n^2 + n) = sum(n^2) + sum(n) = [n(n+1)(2n+1)/6] + [n(n+1)/2]. For n=30: [30*31*61/6] + [30*31/2] = 9455 + 465 = 9920.

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