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If an A.P. has a = 1, tn = 20 and sn = 399, then value of n is :

A. 20 B. 32 C. 38 D. 40 Answer: Option C
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Solution(By Apex Team)

$\begin{aligned}S_n&=\frac{1}{2}(a+l)\times n\\ \Rightarrow&399=(1+20)\times\frac{n}{2}\\ \Rightarrow&399\times2=21\times n\\ \Rightarrow& n=399\times\frac{2}{21}\\ \Rightarrow& n=19\times2\\ \Rightarrow& n=38\end{aligned}$
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