1/2*f(1/2)=(1/2)^2+3*(1/2)^3...+(2n-1)*(1/2)^(n+1)
f(1/2)-1/2*f(1/2)=1/2+2*(1/2)^2+2*(1/2)^3+...+2*(1/2)^n-(2n-1)*(1/2)^(n+1)
可得1/2*f(1/2)=1/2+1/2+(1/2)^2+(1/2)^3+……+(1/2)^(n-1)-(2n-1)*(1/2)^(n+1)
即
f(1/2)=[1+(1/2)^2+(1/2)^3+……+(1/2)^(n-1)-(2n-1)*(1/2)^(n+1)]*2
=2+[(1/2)+(1/2)^2+……+(1/2)^(n-2)-(2n-1)*(1/2)^n]
而[(1/2)+(1/2)^2+……+(1/2)^(n-2)]lt;1
故f(1/2)lt;3