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

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

Answer»

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.

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