Question

If in an AP a= 1 aₙ=20 and Sₙ=399 then the value of n is​

Answer 1

Answer:

a= 1

an= 20

Sn= 399

Sn = n/2 (a+ an)

399 = n/2 ( 1+20)

399×2= n (21)

798= 21n

n= 798/21

n=38

therefore, value of n= 38