Question

Which term of an A.P. 5,9,13,17…. is 81.
(A)15
(B)12
(C)20
(D)None of these.​

Answer 1

(C) 20

Step-by-step explanation:

Given AP is 5,9,13,17,.....

First term is a=5

Common difference 9−5=4

nth term is a+(n−1)d

5+(n−1)4=81

4(n−1)=81−5

4(n−1)=76

n−1=19

n=20

So the 20th term is 81

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Answer 2

Answer:

(C)20

Step-by-step explanation:

given A.P =5,9,13,17….81

here a=5,d=4,An=81,

find: n

solution: An=a+(n-1)d

81 = 5+(n-1)4

81-5=(n-1)4

76=(n-1)4

76 =(n-1)

4

19 =n-1

19+1=n

20=n

so 81 is the 20th term of the A.P

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