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: C. 216

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  1. Step 1: Apply the arithmetic series sum formula .
  2. Step 2: Substitute , , and : .
  3. Step 3: Computing the expression yields .
Answer
216.
Why not the others: '212' evaluates the summation using terms instead of ; '225' omits the factor of in the series formula; '396' adds rather than subtracts the common difference term.

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