Find the sum of an arithmetic series
Add the first and last term, multiply by the number of terms, and halve it.
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Find the sum of the first 6 terms of the arithmetic sequence −12, −9, −6, …
Answer: -27
Find the sum of the first 8 terms of the arithmetic sequence 4, −3, −10, …
Answer: -164
Find the sum of the first 9 terms of the arithmetic sequence −13, −12, −11, …
Answer: -81
Tip: Add the first and last term, multiply by the number of terms, and halve it.
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