why is \$P^{max}_{Z_L}=|\hat I_L|^2\times 4\$,not \$|\hat I_L|^2\times 5\$,when \$\hat Z_L=\hat Z^*_{th}=4-3jΩ\$
25 A flows through a 4 ohm resistance in series with a 3 ohm reactance. Only the resistance dissipates power, hence the power is \$25^2\times 4\$ W.