Question

(i) The first term of an A.P. is 1 and the nth term is 20. If Sn=399, what is n?
(A) 42 (B) 38 (C) 21 (D) 19


Step by step explanation please.​

Answer 1

Answer:

The value of n is 38.

Step-by-step explanation:

Given : The first term of an A.P. is 1 and the nth term is 20. If Sn = 399

To find : The value of n ?

Solution :

The sum of n terms of A.P is given by,

S_n=\frac{n}{2}[a+l]

Where, a is the first term a=1

l is the nth term l=20

S_n=399 is the sum of n terms

Substitute the value in the formula,

399=\frac{n}{2}[1+20]

798=21n

n=\frac{798}{21}

n=38

Therefore, the value of n is 38.

#Learn more

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