An arithmetic sequence has first term -4 and common difference 3. Find the sum of its first 8 terms.
Answer and explanation
Correct answer: A. 52
- Step 1: The last term is a_n = -4 + (8 - 1)(3) = 17.
- Step 2: Use S_n = n(a_1 + a_n)/2 = 8(-4 + 17)/2 = 52.
Answer
52.
Why not the others: '136' uses n(n - 1)d instead of n(n - 1)d/2 in the sum formula; '104' forgets to divide the sum of first and last terms by 2; '55' adds one extra common difference.
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