$ 解：(1)R_{ L}=\frac {U_{ 额 }^2}{P_{ 额 }}=\frac {({ 9 }\ \text {V})^2}{{ 9 }\ \text {W}}={9 }Ω$
$ (2)W_{实 }=\frac {U_{ 实 }^2}{R_{ L }}t_{ }=\frac {({6 }\ \text {V})^2}{{ 9 }Ω}×{ 60 }\ \text {s}={ 240 }\ \text {J}$
$ (3)U_{ }=I_{ }R_{ 总 }={ 3 }\ \text {A}×\frac {\frac R2(\frac R2+9Ω)}{\frac R2+(\frac R2+9Ω)}①$
$ U=\frac 23×(9\ \mathrm {Ω}+R)②$
$ 联立①②，得U=18\ \text {V}，R=18Ω$
$ I_1=\frac {18\ \text {V}}{9Ω+\frac {18}2Ω}=1\ \text {A}$
$ I_{ 2 }=I_{ }-I_{1 }={ 3 }\ \text {A}-{ 1 }\ \text {A}={ 2 }\ \text {A}$
$ P=P_1+P_2=(1\ \text {A})^2×9Ω+(2\ \text {A})^2×9Ω=45\ \text {W}$
