Q. 105.0( 8 Votes )

In an AP, if a = 1, an = 20 and Sn = 399, then the what is the value of n?

Answer :

Given:

First term, a = 1,


Nth term, an = 20,


Some of n terms, Sn = 299


To find: The value of n.


Formula Used:


Sum of “n” terms of an AP:



Where Sn is the sum of first n terms


n = no of terms


a = first term


l = last term


Explanation:


Using the formula



n = no of terms


l = an = last term


2Sn = n (a + an)


2(399) = n (1 + 20)



n = 38


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