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Q.

If the nth terms of the two APs: 9, 7, 5, ... and 24, 21, 18, … are the same, the value of n is[[1]].


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Detailed Solution

If the nth terms of the two APs: 9, 7, 5, ... and 24, 21, 18, … are the same, the value of n is 16.
The given series are 9, 7, 5, ... and 24, 21, 18, … .
From the first series,
a=9  ,
d =79 d =2  
For  a n  , let’s put the values of a and d,
  a n =a+(n1)d =9+(n1)(2) =92n+2 =112n  
From the second series,
a 24  ,
d=2124 d=3  
For a n  , let’s put the values of a and d,
a n =24+ n1 3 =243 n1 =243n+3 =273n  
Now, let’s equate both the last terms,   a n = a n 273n=112n 3n+2n=1127 n=16  
n=16  
For the value of a 16   for the first AP.  a n =a+ n1 d  
a 16 =9+ 161 2 =915 2 =930 =21  
For the value of a 16   for the second A.P.
a 16 =24+ 161 3 =2415 3 =2445 =21  
Thus, the correct answer is 16.
 
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