An arithmetic sequence has first term 4 and common difference 3. Find the sum of its first 12 terms.

Answer and explanation

Correct answer: A. 246

  1. Step 1: The last term is a_n = 4 + (12 - 1)(3) = 37.
  2. Step 2: Use S_n = n(a_1 + a_n)/2 = 12(4 + 37)/2 = 246.
Answer
246.
Why not the others: '444' uses n(n - 1)d instead of n(n - 1)d/2 in the sum formula; '492' forgets to divide the sum of first and last terms by 2; '37' gives the 12th term instead of the sum.

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