An arithmetic sequence has first term 5 and common difference 2. Find the sum of its first 12 terms.
Answer and explanation
Correct answer: C. 192
- Step 1: The last term is a_n = 5 + (12 - 1)(2) = 27.
- Step 2: Use S_n = n(a_1 + a_n)/2 = 12(5 + 27)/2 = 192.
Answer
192.
Why not the others: '324' uses n(n - 1)d instead of n(n - 1)d/2 in the sum formula; '384' forgets to divide the sum of first and last terms by 2; '194' adds one extra common difference.
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