The first term of an arithmetic progression is 9 and the common difference is 5. Find the sum of the first 15 terms.
Answer and explanation
Correct answer: D. 660
- Step 1: Use S = n/2 [2a + (n - 1)d] = 15/2 x [18 + 14 x 5]. Step 2. = 660. Answer: 660. Why not the others: 79: That is only the 15th term, not the sum. 665: That is off by one common difference. 655: That is off by one common difference in the other direction.
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