Q. 42

Find the number o

Answer :

Let the first term and common difference of given AP be a and d respectively.

first term(a) = 18 and,


common difference(d) = (n+1)th term - nth term = (31/2)-18


= -(5/2)


last term or nth term(an) = -(99/2)


Let the total no. of terms in above A.P be n.


an = a+(n-1)×d


-(99/2) = 18+(n-1)×(-5/2)


-99 = 36-5(n-1)


-135 = -5n+5


5n = 140


n = 28


Thus, the no. of terms of the given A.P = 28.

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