`a.`Sơ đồ \(\Leftrightarrow R_1nt\left(R_2\backslash\backslash R_3\right)\)
\(\Rightarrow I=I_1=I_{23}=1,5A\)
\(U_1=I_1.R_1=1,5.3=4,5V\)
\(\Rightarrow U_{23}=U_{AB}-U_1=12-4,5=7,5V\)
\(R_{23}=\dfrac{U_{23}}{I_{23}}=\dfrac{7,5}{1,5}=5\Omega\)
\(R_{23}=\dfrac{R_2.R_3}{R_2+R_3}=5\)
\(\Leftrightarrow\dfrac{10.R_3}{10+R_3}=5\) \(\Leftrightarrow R_3=10\Omega\)
`b.` Sơ đồ \(\Leftrightarrow R_xnt\left(R_2\backslash\backslash R_3\right)\)
\(\Rightarrow I=I_x=I_{23}=0,4A\)
\(U_{23}=I_{23}.R_{23}=0,4.5=2V\)
\(U_x=U_{AB}-U_{23}=12-2=10V\)
\(R_x=\dfrac{U_x}{I_x}=\dfrac{10}{0,4}=25\Omega\)
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