In an arithmetic sequence, the 4th term is and the 8th term is . Find the sum of the first terms of this sequence.
Answer and explanation
Correct answer: B.
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Subtract the first equation from the second: .
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Subtract the first equation from the second: .
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- Step 1: Express the 4th and 8th terms in terms of the first term and common difference :
- Step 2: Find the first term :
- Step 3: Compute the sum of the first terms using :
Answer
.
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