Question

The value of sum is –

• A. 5
• B. 4
• C. 2
• D. None of these

Answer 1

Answer: (C)

2

Answer 2

$\sum _ { k = 1 } ^ { \infty } \frac { \mathrm { k } } { 2 ^ { \mathrm { k } } } \left( \sum _ { \mathrm { n } = 1 } ^ { \infty } \frac { 1 } { 2 ^ { \mathrm { n } } } \right)$ =

= $\frac { 1 } { 2 }$+ $\frac { 3 } { 2 ^ { 3 } }$.....

which is A.G.P. & can be solved.