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Question

Which term of the AP = 21, 18, 15, ... is -81?
 


Solution

a = 21 

d = 18 - 21 = -3 .

Let -81 be the nth term. 


Now, 
 nth term,    an=a+(n1)d

-81 = 21 + ( n -1 ) -3 

-81 = 21 -3n + 3 

-81 = 24-3n 

3n = 24 + 81
n = 1053


n=35

Therefore, the 35th term is -81.

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