In the given Fig. the Thevenin's equivalent pair (voltage, impedence), as seen at the terminals $$P-Q$$, is given by

Two ac sources feed a common variable resistive load as shown in Fig. Under the maximum power transfer condition, the power absorbed by the load resistance $${R_L}$$ is

In Fig. the potential difference between points $$P$$ and $$Q$$ is

Viewed from the terminals $$A, B$$ the following circuit shown in Figure can be reduced to an equivalent circuit of a single voltage source in series with a single resistor with the following parameters: