In the small-signal analysis of a vacuum tube circuit, which network theorem is most generally and frequently used?

In a linear network, if the open-circuit voltage across two terminals is 8 V and the short-circuit current through them is 2 A, determine the Thevenin's equivalent voltage (VTh) and impedance (ZTh).

In a given network, the open-circuit voltage across terminals is Vth = 24 ∠0° V and the short-circuit current is Isc = 1.8 ∠14.5° A. What is the Thevenin equivalent impedance (Zth) of the circuit?