An arithmetic progression has first term and common difference . Find the sum of the first 12 terms.

Answer and explanation

Correct answer: D. -78

  1. Step 1: . Step 2. . Answer: -78. Why not the others: -276: That multiplies the last term by instead of using the average of the first and last. -23: That is only the 12th term. -81: That is off by one common difference.

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