An arithmetic sequence has first term 9 and common difference 2. Find the sum of its first 15 terms.

Answer and explanation

Correct answer: D. 345

  1. Step 1: The last term is a_n = 9 + (15 - 1)(2) = 37.
  2. Step 2: Use S_n = n(a_1 + a_n)/2 = 15(9 + 37)/2 = 345.
Answer
345.
Why not the others: '555' uses n(n - 1)d instead of n(n - 1)d/2 in the sum formula; '690' forgets to divide the sum of first and last terms by 2; '37' gives the 15th term instead of the sum.

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