解：?$(1)P_{ 額 }=U_{ 額 }I_{額 }={ 6 }\ \text{V}×{ 0.8 }\ \text{A}={ 4.8 }\ \text{W}$?
?$ (2)R_2=2R_1，$?
所以?$U_1=\frac 13U，$??$P_1∶P_1'=U_1^2∶U_1'^2=3\ \text {W}∶0.75\ \text {W}=4，$?
所以?$U_1=2U_1'，$?即?$\frac 13U=2×\frac {R_1}{R_1+R_L}U，$?得?$R_L=5R_1$?
所以?$P_L=5P_1'=5×0.75\ \text {W}=3.75\ \text {W}$?
