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The sum of a 15-term arithmetic series with first term 7 is -210. What is the common difference?

 Aug 20, 2020
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Use the formula for summing up an arithmetic sequence:
15/2 * [2*7 + 14D] = - 210, solve for D
7.5 * [14 + 14D] = - 210
105 + 105D = -210
105D = - 210 - 105
105D = - 315
D = - 315 / 105
D = - 3  the common difference

 Aug 20, 2020

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