Question

If an AP if a =1,an =20 and Sn =399, then n is (a) 19,(b) 21,(c) 38,(d) 42​

Answer 1

sn=n/2[a+an]
399=n/2[1+20]
2(399)=n(21)
798=21n
n=38
hence n is (c)38

Answer 2

option ( c ) is ryt

a = 1,an = 20, sn = 399

we know that

sn = n/2[a + an]

=> 399 = n/2[ 1 + 20 ]

=> 399*2 = 21n

=> 798 = 21n

=> n = 798/21

=> n = 38