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Which term of the arithmetic progression 21, 42, 63, 84 _____ is 420
1. 21
2. 19
3. 20
4. 22

1 Answer

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Best answer
Correct Answer - Option 3 : 20

Given: 

A.P is 21 ,42, 63 ,84, .........

Concept: 

The formula for finding the n-th term of an AP is:

an = a1 + (n - 1) d 

Calculation:

a1 = 21 , a = 42 , a3  = 63 ,a = 84 ......

⇒ d = a2 - a1 = 42 - 21 = 21

Let 420 be then nth term using an = a1 + (n - 1) d

⇒ 21 + 21n - 21 = 420

⇒ 21n = 420

⇒ n = 20

∴ 20th term is 420

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