Q. 1 J4.5( 10 Votes )

# Choose the correct alternative answer for each of the following sub questions.

In an A.P. 1st term is 1 and the last term is 20. The sum of all terms is = 399 then n = . . .

A. 42

B. 38

C. 21

D. 19

Answer :

Given, first term = 1

Last term = 20

Sum of n terms, S_{n} = 399

We need to find no. of terms n

Using Sum of n terms of an A.P. formula

where n = no. of terms

S_{n} = sum of n terms

Now, on substituting given value in formula we get,

∴ correct answer is (B)

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