Answer :

In the given AP, the first term = a = 18

Common difference = d = (31/2) - 18 = (-5/2)


Last term = - 47


To find: No. of terms in the AP.


Since, we know that


an = a + (n - 1) × d


- 47 = 18 + (n - 1) × (-5/2)


- 47 - 18= (n - 1) × (-5/2)


- 65 = (n - 1) × (-5/2)


- 65 × (-2/5) = n - 1


n - 1 = 26


n = 26 + 1


n = 27


Number of terms = 27.


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