Q. 54.1( 228 Votes )

Find the number of terms in each of the following APs :

(i) 7, 13, 19, . . . , 205

(ii)

Answer :

To find: Number of terms, n
(i) Given:  a = 7, d = 6, an = 205, n = ?


We know that nth term of an AP is given by,  an = a + (n – 1)d


205 = 7 + (n – 1)6


(n – 1)6 = 205 – 7 = 198


n – 1 = 33


n = 34


Thus, 205 is the 34th of term of this AP.


(ii) Given: a = 18, d = 15.5 – 18 = - 2.5, an = - 47


We know that nth term of an AP is given by,  an = a + (n – 1)d


Or, - 47 = 18 + (n – 1)(- 2.5)


Or, (n – 1)(- 2.5) = - 47 – 18 = - 65


Or, n – 1 = 26

Or, n = 27

Thus, - 47 is the 27th term of this AP.

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