After the power supply is reset to zero, it is easy to know that the equivalent resistance of the circuit at the ab terminal is 30∑60 = 20 ohms.

Then the open circuit voltage is calculated by superposition principle. Disconnect the current source first, and the VCR knows that U'ab=30x30/(30+60)= 10v, then short-circuit the voltage source and reset the current source. The VCR knows that U' AB = 30x60/(30+60) = 20V, so UAB = U 'AB+U 'AB =

To sum up, according to Thevenin theorem, the original circuit can be equivalent to a 30v voltage source and a 20Ohm resistor connected in series with a two-terminal network.

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