{an} is an arithmetic sequence A3 = 12 S12 > 0 S13
Because in the arithmetic sequence, there is S13 = 13 * A70 (2)
In the arithmetic sequence, there are A6 = A3 + 3D = 12 + 3D, a7 = A3 + 4D = 12 + 4D
a7=12+4d-24/7.
So the range of D is - 24 / 70, so n
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