Question

Sum to n terms

(3n – 1) + 2. (3n – 2) + 3.(3n – 3) + .............n terms is

(a)  n(2n + 1)(5n – 1)/3

(b)  n(2n + 1)(5n + 1)/3

(c)   n(n + 1)(7n – 1)/6

(d)  None of these. 

Answer 1

Correct option (c) n(n + 1)(7n – 1)/6

Explanation:

T_r = r(3n–r)

= 3nr–r²

= n(n + 1)(7n – 1)/6