SOLUTION: The first term of an AP is 7, the last term is 77 and the sum is 380. Find the common difference in the series.

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Question 861184: The first term of an AP is 7, the last term is 77 and the sum is 380. Find the common difference in the series.
Answer by richwmiller(17219) About Me  (Show Source):
You can put this solution on YOUR website!
S = ½(a + an)n
380= ½(7 + 77)n
380= ½(84)n
380=42n
380/42=n
190/21=n
n = 9.0476
not possible
The number of terms is not a whole number