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  • In an AP if a = 1, an = 20 and Sn = 399, then n is (A) 19 (B) 21 (C) 38 (D) 42

\text{In an AP if a = 1,} a\textsubscript{n} = 20 and S\textsubscript{n} = 399\text{, then n is }

(A) 19

(B) 21

(C) 38

(D) 42

Answers (1)

\\$Given : a = 1, a\textsubscript{n} = 20, S\textsubscript{n} = 399\\$ {{S}_{n}}=\frac{n}{2}(a+{{a}_{n}}) \\ 38 = n\\

Hence option C is correct.

Posted by

infoexpert21

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