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The value of n, when a=1, $a n$ = 20 and $S n = 399$ is

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answer is C.

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Detailed Solution

Given,

a = 1

a n = 20

and

S n = 399

.
So, according to the question, by using the formula we will find no. of terms.

S n = n 2 a + a n

399 = n 2 1 + 20 39921 = n 2 n = 192 n = 38

So, n=38.
Therefore, the correct option is 3.

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