Discussion
The first term of an A.P. is 5, the last term is 45, and the sum is 400. Find the number of terms.
Answer: C
We use the formula for the sum of an A.P. when the first and last terms are known: \(S_n = \frac{n}{2}(a+l)\). Given \(S_n=400, a=5, l=45\). \(400 = \frac{n}{2}(5+45) \Rightarrow 400 = \frac{n}{2}(50) \Rightarrow 400 = 25n \Rightarrow n = 400/25 = 16\).
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