Q. 234.8( 4 Votes )

The sum of first

Answer :

Sum of first n terms = 5n2 + 3n

Putting n = 1, we get


S1 = 5(1)2 + 3(1) = 8


Putting n = 2, we get


Sum of first 2 terms = S2 = 5(2)2 + 3(2) = 26


The second term = 26 – 8 = 18


Putting n = 3, we get


S3 = 5(3)2 + 3(3) = 54


The third term = 54 – 26 = 28


First term = a = 8


Common difference = d = 10


The formula for nth term of an A.P. is given by,


an = a + (n – 1) d


mth term = am = a + (m – 1) d = 168


8 + (m – 1) 10 = 168


8 + 10m – 10 = 168


10m = 170


m = 17


a20 = 8 + (20 – 1) 10


= 198


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