If Sn, the sum of first n terms of an A. P., is given by Sn = 3n^2 -4n, then find its nth term

 Solution

We know that 

Sn - Sn-1=an 

=3n^2 -4n-( 3(n-1)^2 -4(n-1) 

=3n^2 -4n-( 3n^2-6n+3 -4n+4)

=3n^2 -4n-3n^2+6n-3+4n-4

=3n^2 -4n -3n^2+6n-3+4n-4

=6n-7 

nth term of an A.P=6n-7 

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