The first term of an arithmetic sequence is 7 and the common difference is 4. What is the sum of the first 20 terms?
Answer and explanation
Correct answer: D. 900
- Step 1: S_n = n(2a_1 + (n - 1)d) / 2.
- Step 2: S_20 = 20(14 + 76) / 2 = 900.
Answer
900.
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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