Find the sum of the first 20 terms in an arithmetic progression whose first term is 7 and last term is 117.
Given the first and last term of an A.P, the sum of the terms is given by
\(S_{n} = \frac{n}{2} [a + l]\)
where a = first term; l = last term and n = number of terms.
\(\therefore S_{20} = \frac{20}{2} [7 + 117]\)
= \(10 (124)\)
= 1240
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