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Sequences and series
Use the formula: 1/2 n [2a+(n-1)d]
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An arithmetic series has the first term -3 and common difference d. The sum of the first 20 terms of the series is 320. Find the value of d.
1/2 (20)[2(-3) + (20 -1)d] = 320 10(-6+19d) = 320 -6+19d = 32 19d = 38 d=2
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