An arithmetic sequence has first term -2 and common difference 4. Find the sum of its first 9 terms.

Answer and explanation

Correct answer: C. 126

  1. Step 1: The last term is a_n = -2 + (9 - 1)(4) = 30.
  2. Step 2: Use S_n = n(a_1 + a_n)/2 = 9(-2 + 30)/2 = 126.
Answer
126.
Why not the others: '270' uses n(n - 1)d instead of n(n - 1)d/2 in the sum formula; '252' forgets to divide the sum of first and last terms by 2; '30' gives the 9th term instead of the sum.

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