The first term of an arithmetic progression is and the common difference is . Find the sum of the first terms.

Answer and explanation

Correct answer: D.

  1. Step 1: Use .
  2. Step 2: .
Answer
.
Why not the others: is what you get if you use in place of . is what you get if you forget to halve. is what you get if you put instead of in the formula.

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