Question

. In an AP if a = 1, aₙ= 20 and Sₙ = 399, then n is​

Answer 1

Step-by-step explanation:

399 is the answer

is it right

Answer 2

Answer:

a= 1

an =20

sn=399

SN= 1/2 (2a+(n-1)d

so,

sn=1/2 (2a+an)

399=1/2 (2 n+20)

.......

solve yourself