An arithmetic sequence has first term 10 and common difference -2. Find the sum of its first 9 terms.
Answer and explanation
Correct answer: C. 18
- Step 1: The last term is a_n = 10 + (9 - 1)(-2) = -6.
- Step 2: Use S_n = n(a_1 + a_n)/2 = 9(10 + -6)/2 = 18.
Answer
18.
Why not the others: '-54' uses n(n - 1)d instead of n(n - 1)d/2 in the sum formula; '36' forgets to divide the sum of first and last terms by 2; '16' adds one extra common difference.
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