An arithmetic sequence has first term 3 and common difference 4. What is the sum of its first 10 terms?
Answer and explanation
Correct answer: C. 210, from 10/2 times (2 x 3 + 9 x 4)
- Step 1: Use S = n/2 x (2a + (n - 1)d) with n = 10, a = 3, d = 4. Step 2. S = 5 x (6 + 36) = 5 x 42 = 210. Answer: 210. Why not the others: The others use the wrong number of differences or forget the factor n/2.
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