Question

if an AP a=1 an=20 sn=390 then n is​

Answer 1

Answer:

a=1

An=nth term=20

1+(n-1)d=20

d=19/(n-1)

Sum of nth term=390

n/2[2×1+(n-1)d]=390

n[2+(n-1)(19/n-1)]=390×2

n[2+19]=780

n=780/21=260/21