Final answer:
The voltage across the resistor in a circuit with a diode connected in series can be found by subtracting the forward voltage of the diode from the total voltage supplied by the power source.
Step-by-step explanation:
In a circuit where a diode is connected in series with a resistor, the voltage across the resistor can be found by subtracting the forward voltage of the diode from the total voltage supplied by the power source. If the forward voltage across the diode is 0.8 volts and the power source provides a voltage of 4 volts, then the voltage across the resistor would be 4 - 0.8 = 3.2 volts.