Answer :

In the given AP, the first term = a = 21

Common difference = d = 18 - 21 = - 3


To find: place of the term - 81.


So, let an = - 81


Since, we know that


an = a + (n - 1) × d


- 81 = 21 + (n - 1) × (-3)


- 81 - 21 = - 3n + 3


- 102 = - 3n + 3


- 102 - 3 = - 3n


- 3n = - 105


n = 105/3


n = 35


35th term of the AP is - 81.


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