An arithmetic sequence has first term 8 and common difference -3. Find the sum of its first 10 terms.

Answer and explanation

Correct answer: B. -55

  1. Step 1: The last term is a_n = 8 + (10 - 1)(-3) = -19.
  2. Step 2: Use S_n = n(a_1 + a_n)/2 = 10(8 + -19)/2 = -55.
Answer
-55.
Why not the others: '-190' uses n(n - 1)d instead of n(n - 1)d/2 in the sum formula; '-110' forgets to divide the sum of first and last terms by 2; '-19' gives the 10th term instead of the sum.

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