\(a)R_{tđ}=\dfrac{1}{R_1}+\dfrac{1}{R_2}+\dfrac{1}{R_3}\\ \Leftrightarrow R_{tđ}=\dfrac{1}{30}+\dfrac{1}{30}+\dfrac{1}{30}\\ \Leftrightarrow R_{tđ}=10\Omega\\ b)U_1=I_1.R_1=30.0,6=18V\\ R_1//R_2//R_3\\ \Rightarrow U=U_1=U_2=U_3=18V\\ I_2=\dfrac{U_2}{R_2}=\dfrac{18}{30}=0,6A\\ I_3=\dfrac{U_2}{R_2}=\dfrac{18}{30}=0,6A\)