Question

Which term of the AP 5 , 9 , 13 , 17 .. is 81

Answer 1

n = 20 by a=5&d=4 so, an = a+(n-1)d

Answer 2

Let a be the first term and d be the common difference.

Here, first term a = 5, Common difference d = 9 - 5 = 4, an = 81.

We know that nth term of an AP : an = a + (n - 1) * d

= > 81 = 5 + (n - 1) * 4

= > 81 = 5 + 4n - 4

= > 81 = 4n + 1

= > 80 = 4n

= > n = 20.

Therefore, the result is 20th term.

Hope this helps!