Question

n In an A.P., if a = 1, tn = 20 and Sn = 462, then n is (A) 42 (B) 29 (C) 39 (D) 44​

Answer 1

Answer:

Step-by-step explanation:

Given that,

a = 1 , tn = 20 and Sn = 462

tn = a + ( n - 1 )d

20 = 1 + ( n - 1 ) d

20 - 1 = ( n - 1 ) d

( n - 1 ) d = 19

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

462 = n/2 [ 2 ( 1 ) + 19 ] ( ∵ ( n - 1 )d = 19 )

462 × 2 = n [2+ 19 ]

924 = n [ 21 ]

n = 924 / 21

n = 44 is the answer.