The first term of an arithmetic sequence is 5 and the common difference is -3. What is the sum of the first 12 terms?
Answer and explanation
Correct answer: C. -138
- Step 1: S_n = n(2a_1 + (n - 1)d) / 2.
- Step 2: S_12 = 12(10 + -33) / 2 = -138.
Answer
-138.
Why not the others: The other values use one term too few, forget to halve, or use n times the last term.
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