Question

In an A.P,if a=1,an=20 and Sn=399 ,then n=____

Answer 1

Answer:

a=1

an=20

a+(n-1)d=20

1+(n-1)d=20

d(n-1)=19

d=19/(n-1)

Sn=399.

n[2a+(n-1)d]=399×2

n[2x1+(n-1)×{19/(n-1)}]=399×2

n[2+19]=399×2

21n=399×2

n=19×2

n=38

Answer 2

Sn = n/2[a+an]

given :-

a=1 , an=20 & Sn = 399

putting value:

399=n/2[1+20]

798 = 21n

n = 798/21

n= 38 ans

hope it'll help you..