解：$(1)R_L=\frac {U_L^2}{P_L}=\frac {(12\ \text {V})^2}{6\ \text {W}}=24 \ Ω$
$(2)U=U_L=12\ \text {V}$
$I_L=\frac {P_L}{U_L}=\frac {6\ \text {W}}{12\ \text {V}}=0.5\ \text {A}$
$I_{1}=I-I_L=0.9\ \text {A}-0.5\ \text {A}=0.4\ \text {A}$
$R_{1}=\frac {U}{I_{1}}=\frac {12\ \text {V}}{0.4\ \text {A}}=30 \ Ω$
$(3)I_{min }=\frac {U_{ }}{R_{L }+R_{2}}=\frac { 12\ \text {V}}{12 \ Ω+12 \ Ω}=0.5\ \text {A}$
$P_{Lmin}=I_{min}^2R_L=(0.25\ \text {A})^2×24 \ Ω=1.5\ \text {W}$
