Question

If an AP, a=1,an=20,Sn =399
then n'is​

Answer 1

Answer:

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