SOLUTION: the first term of an A.P is 7,the last term is 70 and sum is 385.find the number of terms in the series

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Question 1124731: the first term of an A.P is 7,the last term is 70 and sum is 385.find the number of terms in the series

Answer by greenestamps(13200) About Me  (Show Source):
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The sum of an arithmetic progression is the number of terms, multiplied by the average of the first and last terms:

S+=+n%28%28a%281%29%2Ba%28n%29%29%2F2%29

Plug in the given information:

385+=+n%28%287%2B70%29%2F2%29
770+=+77n
n+=+10

There are 10 terms in the A. P.