a) \(R_{tđ}=R_1+R_2=15+25=40\left(\Omega\right)\)
b) \(I=I_1=I_2=\dfrac{U}{R_{tđ}}=\dfrac{9}{40}\left(A\right)\)
c) \(\left\{{}\begin{matrix}U_1=I_1.R_1=\dfrac{9}{40}.15=\dfrac{27}{8}\left(V\right)\\U_2=I_2.R_2=\dfrac{9}{40}.25=\dfrac{45}{8}\left(V\right)\end{matrix}\right.\)