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The sum of the first four terms of an AP is 2/3 and the sum of the first eight term is -1.find the sum of the first twelve terms

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S4 = 4/2 (2a+3d) = 2/3
S8 = 8/2 (2a+7d) = -1
a = 37/96 and d = -7/48
S12 = 12/2 (37/48 + 11(-7/48)) = -5

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