1/2 * sn = 1/2+2* 1/2^2+3* 1/2^3+……+(n- 1)* 1/2^(n- 1)+n* 1/2^n

The result of dislocation subtraction is1/2 * sn =1+1/2+1+1/2 (n-1)-n */kloc-.

So sn = 2+1+1/2+...+1/2 (n-2)-n *1/2 (n-1).

So sn = 2+1* (1-kloc-0//2 (n-1))/(1-kloc-0//2)-n *1/.

Sn=4-(n+2)/2^(n- 1)

This problem belongs to differential sequence, and the commonly used method is dislocation subtraction.

Step: 1. By sharing common proportions.

2. Dislocation subtraction

3. Sort out the equation.