I'm having a really hard time understanding input vs. output impedance and also the usefulness of the Thevenin model. Let's say we have a simple voltage divider consisting of two equal resistors. The output voltage is between the resistors. If the input voltage is 10v the output voltage has to be 5v. What is the point of the Thevenin resistor then? I know that the Thevenin resistor is the output impedance of the circuit but I don't understand why. Also, for a random circuit, how do you calculate the input impedance and output impedance, and how does this relate to "looking into" a circuit?