f'(x)=-2/(1-2x)=2/(2x-1)=1/(x-1/2)
f''(x)=-1/(x-1/2)^2
f'''(x)=2 /(x-1/2)^3
...
fn'(x)=(-1)^(n+1) *(n-1)!*(x-1/2)^(-n)