[tex]\displaystyle E=4,5~V;~R=10~\Omega;~U=4~V\\ \\ a)~U=IR\Rightarrow I=\frac{U}{R} \Rightarrow I=\frac{4}{10} \Rightarrow \mathbf{I=0,4~A}\\ \\ b)~E=U+u\Rightarrow -u=-E+U\Rightarrow u=E-U \Rightarrow u=4,5-4 \Rightarrow \mathbf{u=0,5~V}\\ \\ c)~u=Ir\Rightarrow r=\frac{u}{I} \Rightarrow r=\frac{0,5}{0,4} \Rightarrow \mathbf{r=1,25~\Omega}[/tex]