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: B. 560

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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
560.
Why not the others: '558' evaluates the summation using terms instead of ; '980' omits the factor of in the series formula; '580' uses instead of for the number of difference steps.

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