sentences:  is the double-null-injection driving point impedance "seen" by the extra element.The extra element theorem incidentally proves that any electric circuit transfer function can be expressed as no more than a bilinear function of any particular circuit element.is found by making the input to the system's transfer function zero (short circuit a voltage source or open circuit a current source) and determining the impedance across the terminals to which the extra element will be connected with the extra element absent.This impedance is same as the Thévenin's equivalent impedance.