Zero voltage source means that the voltage at both ends of the voltage source is 0V, that is, the voltage source is regarded as a short circuit.
Zero current source means that the current flowing through the current source is 0A, that is, the current source is regarded as an open circuit.
2. The situation that the open circuit short circuit method cannot be used is that there is a dependent power supply in the original network, that is, there is a controlled source.
At this time, a power supply with a voltage of U should be set at the opening of the two-terminal network, and then the current I flowing through the overvoltage source can be obtained according to Kirchhoff's voltage-current law, that is, I is an expression of U, and finally the equivalent resistance can be obtained according to R = U/I.
3, no pictures, not all people who answer questions online will buy a book because of a question.