(z-1)(z-2)/(z-3)(z-4) =1+6/(z-4)-2/(z-3)
=1+(-6)/(4(1-z/4))-2/(z(1-3/z))
=-1/2-∑(n=1到∞)6z^n/4^(n+1)-∑(n=1到∞)2*3^(n-1)/z^n