Question

5. In an A.P. A1, A2, a3,... A11..., if a1 = 1, an, = 20 and Sn = 399, then the value of n is

​

Answer 1

Answer:

Correct option is

C

38

We know that S

n

=

2

n

[2a+(n−1)d]

⇒S

n

=

2

n

[a+a+(n−1)d]

⇒399=

2

n

[a+a

n

] [a

n

= last term]

⇒399=

2

n

[1+20]

⇒n=

21

399×2

=38