The first term of an arithmetic progression is and its common difference is . What is the sum of its first terms?

Answer and explanation

Correct answer: D. 390

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  1. Step 1: Recall the standard summation identity for an arithmetic sequence: .
  2. Step 2: Insert the given values , , and : .
  3. Step 3: Evaluating the bracketed term gives , so .
Answer
390.
Why not the others: '386' evaluates the summation using terms instead of ; '403' omits the factor of in the series formula; '754' adds rather than subtracts the common difference term.

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