Question

In an arithmetic series the 3rd term is -8 and 12th term is -35. Determine the number of terms in the series if its sum is -100
a+2d = -8
a+11d = -35
so, 9d = -27
Now you can find a and d, and then solve
n/2 (2a+(n-1)d) = -100
n = 8

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