(1/xk ) - (1+ x-k ) / ( 1-xk )
Hauptnenner xk * ( 1-xk ) also
= ( ( 1-xk ) - xk * (1+ x-k ) ) / ( xk * ( 1-xk ) )
= ( 1-xk - xk - x0) ) / ( xk * ( 1-xk ) )
= ( 1-xk - xk - 1) ) / ( xk * ( 1-xk ) )
= ( -2xk ) / ( xk * ( 1-xk ) ) kürzen
= -2 / ( 1-xk )
= 2 / ( xk - 1 )