Question

a=1 an=20 sn=399 find n?​

Answer 1

Step-by-step explanation:

) Sn = n/2 [2a + (n-1)d]

399 = n/2 [2 × 1 + (n-1)d]

798 = 2n + n(n-1)d ...(i)

and an = 20

⇒ a + (n-1)d = 20 [∵ an = a + (n-1)d]

⇒ 1 + (n-1)d = 20 ⇒ (n-1)d = 19 ...(ii)

Using Eq.(ii) in Eq (i), we get

798 = 2n + 19n

⇒ 798 = 21n

∴ n = 798 / 21 = 38

Answer 2

Answer:

38

Step-by-step explanation:

using formula of Sn=n/2(a+an)

399=n/2(1+20)

n=399*2/21

n=38.