Q. 184.4( 5 Votes )

If the sum of the

Answer :

We know that sum of n terms of an A.P. is given by,



Where a= first term, d= Common difference, n= No. of terms



3n2 + 7n=2an + (n-1)nd


3n2 + 7n=n2d + (2a-d)n


By comparing, we get


d=3,


2a-d=7


2a-3=7


2a=10


a=5


So, nth term of this A.P.,


An=a + (n-1)d=5 + (n-1)3=5 + 3n-3=3n + 2


So,nth term is given by (3n + 2)


Now,For 20th term, n=20


A20=3 × 20 + 2=62


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