An arithmetic sequence has first term 2 and common difference 5. Find the sum of its first 15 terms.
Answer and explanation
Correct answer: B. 555
- Step 1: The last term is a_n = 2 + (15 - 1)(5) = 72.
- Step 2: Use S_n = n(a_1 + a_n)/2 = 15(2 + 72)/2 = 555.
Answer
555.
Why not the others: '1080' uses n(n - 1)d instead of n(n - 1)d/2 in the sum formula; '1110' forgets to divide the sum of first and last terms by 2; '560' adds one extra common difference.
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