Question

in an AP if a=1, a11= 20 and Sn= 399, then 'n' is???? a.19 b.21 c.38 d.42.......... please answer fast..........plsss..........can anyone help me.....plss
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Answer 1

Answer:

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

=

21

399×2

=38 this type answer

Answer 2

hey apoorva sorry for spamming in your question

but can I inbox you I have some questions