The sum of the first 20 terms of the series `1/i+2/i^2+3/i^3+.....` is :

The sum to first 25 terms of the series 1+2+3+"…………" is

The difference between the sum of the first k terms of the series 1^3+2^3+3^3+.......+n^3 and the sum of the first k terms of 1+2+3+.......+n is 1980. The value of k is __________.

The difference between the sum of the first k terms of the series 1^3+2^3+3^3+.......+n^3 and the sum of the first k terms of 1+2+3+.......+n is 1980. The value of k is __________.

The difference between the sum of the first k terms of the series 1^3+2^3+3^3+....+n^3 and the sum of the first k terms of 1+2+3+.....+n is 1980 . The value of k is :

The difference between the sum of the first k terms of the series 1^(3)+2^(3)+3^(3)+...+n^(3) and the sum of the first k terms of 1+2+3+....+n is 1980. The value of k is :

The difference between the sum of the first k terms of the series 1^(3)+2^(3)+3^(3)+...+n^(3) and the sum of the first k terms of 1+2+3+...+n is 1980. The value of k is

The difference between the sum of the first k terms of the series 1^3+2^3+3^3++n^3 and the sum of the first k terms of 1+2+3++ni s1980. The value of k is __________.

Find the sum of first n terms and the sum of first 5 terms of the geometric series : 1+2/3+4/9 + .........