Question

The first term of an arithmetic series is 1 and the nth term is 20 if Sn = 399 then n =?​

Answer 1

Answer:-

Therefore, the value of n is 38.

Step-by-step explanation:

t1=1

tn=20

sn=399

Now,

Sn=n/2*(t1+tn)

399=n/2*(1+20)

399=n/2*21

798=21n

n=798/21

n=38