An arithmetic sequence has first term 7 and common difference 3. Find the sum of its first 14 terms.
Answer and explanation
Correct answer: C. 371
- Step 1: The last term is a_n = 7 + (14 - 1)(3) = 46.
- Step 2: Use S_n = n(a_1 + a_n)/2 = 14(7 + 46)/2 = 371.
Answer
371.
Why not the others: '644' uses n(n - 1)d instead of n(n - 1)d/2 in the sum formula; '742' forgets to divide the sum of first and last terms by 2; '374' adds one extra common difference.
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