设S=2分之1+2的2次方分之1+2的3次方分之1+……+2的n次方分之1
则：S/2=1/2^2+1/2^3+1/2^8+...+1/2^(n+1)
S-S/2=1/2-1/2^(n+1)
S=1-1/2^(n+2)