CTM: \(R_1nt\left(R_2//R_3//R_4\right)\)
\(I_A=I_3+I_4=0,72A;R_{34}=\dfrac{R_3\cdot R_4}{R_3+R_4}=\dfrac{12\cdot R_4}{12+R_4}\)
\(\Rightarrow U_2=U_{34}=0,72\cdot\left(\dfrac{12\cdot R_4}{12+R_4}\right)\Rightarrow U_1=U_{MN}-U_2=18-0,72\cdot\left(\dfrac{12\cdot R_4}{12+R_4}\right)\)
\(I_2=\dfrac{U_2}{R_2}=\dfrac{0,72\cdot\left(\dfrac{12\cdot R_4}{12+R_4}\right)}{4}=0,18\cdot\left(\dfrac{12\cdot R_4}{12+R_4}\right)\)
Ta có: \(I_1=I_{234}=I_2+I_{34}\Rightarrow\dfrac{U_1}{R_1}=I_2+I_A\)
\(\Rightarrow\dfrac{18-0,72\cdot\left(\dfrac{12\cdot R_4}{12+R_4}\right)}{7,6}=0,18\cdot\left(\dfrac{12\cdot R_4}{12+R_4}\right)+0,72\)
\(\Rightarrow R_4=12\Omega\)
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