Question

Which term of the AP 21. 42, 63, 84, … is 420?

Answer 1

Given A. P 21, 42, 63, 84, ………

a = 21, d = a₂ – a₁

= 42 – 21

= 21

We know that, n^th term (a_n) = a +(n – 1)d

And, given n^th term = 420

21 + (n – 1)21 = 420

(n – 1)21 = 399

n – 1 = 399/21 = 19

n = 20

∴ 20^th term is 420.