Question

In an AP, if a=1,an=20 and Sn= 399, then n=?

Answer 1

since ,
an = a+(n-1)d

20 = 1 + (n-1)d

19 = (n-1)d

also ,

Sn = n/2 {2a+19}

399 = n/2(21)

n = 38

Answer 2

given, 
a=1
an=20
sn=399
we know that,
            an=a+(n-1)d
             20=1+(n-1)d
            20-1=(n-1)d
            19= (n-1)d
   sn=n/2{2n+(n-1)d}
   399=n/2{2(1) + (n-1)d}
  798=n(2+ 19)
  798=n(21)
  798/21=n
  38=n