An arithmetic sequence has first term 6 and common difference 5. Find the sum of its first 10 terms.
Answer and explanation
Correct answer: C. 285
- Step 1: The last term is a_n = 6 + (10 - 1)(5) = 51.
- Step 2: Use S_n = n(a_1 + a_n)/2 = 10(6 + 51)/2 = 285.
Answer
285.
Why not the others: '510' uses n(n - 1)d instead of n(n - 1)d/2 in the sum formula; '570' forgets to divide the sum of first and last terms by 2; '51' gives the 10th term instead of the sum.
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