Question

The sum of n term of an ap is 3nsquare+4n find its 25th term

Answer 1

Answer: Sn = 3n^2 + 4n

we know S1 = a

=> a = 3 x 1^2 + 4 x 1

=> a = 3 + 4

=> a = 7

S2 = a + a2

=> 3 x 2^2 + 4 x 2 = 7 + a2

=> 3 x 4 + 8 = 7 +a2

=> 12 + 8 = 7 + a2

=> 20 - 7 = a2

=> a2 = 13

now, d = a2 - a = 13 - 7 = 6

a25 = a + (25-1)d

       = a + 24d

       = 7 + 24 x 6

       = 7 +144

        = 151

hence the 25th term is 151.