Sum of n terms of the following series 13 + 33 + 53 + 73 + ... is
n2(2n2 - 1)
n3(n - 1)
n3 + 8n + 4
2n4 + 3n2
A.
n2(2n2 - 1)
The nth term of given series tn = (2n - 1)3
sum of series =
= 2n2(n + 1)2 - 2n(n + 1)(2n + 1) + 3n(n + 1) - n
= 2n4 + 4n3 + 2n2 - 2n[2n2 + 3n + 1] + 3n2 + 3n - n
= 2n4 + 4n3 + 2n2 - 4n3 - 6n2 - 2n + 3n2 + 3n - n
= 2n4 - n2
= n2(2n2 - 1)
If three positive real numbers a, b, c are in AP and abc = 4, then the minimum possible value of b is 2
If a, b, c are in GP (a> 1, b > 1, c > 1), then for any real number x (with x > 0, x 1), are in
GP
AP
HP
GP but not in HP