a) \(R_{12}=R_1+R_2=15+25=40\left(\Omega\right)\left(R_1ntR_2\right)\)
\(R_{tđ}=\dfrac{R_{12}.R_3}{R_{12}+R_3}=\dfrac{40.10}{40+10}=8\left(\Omega\right)\)(R12//R3)
b) \(U=U_3=U_{12}=12V\)
\(\Rightarrow I_3=\dfrac{U_3}{R_3}=\dfrac{12}{10}=1,2\left(A\right)\)
\(I_{12}=I_1=I_2=\dfrac{U_{12}}{R_{12}}=\dfrac{12}{40}=0,3\left(A\right)\)
c) \(R_{tđ}=\dfrac{R_{x2}.R_3}{R_{x2}+R_3}=7,5\)
\(\Leftrightarrow\dfrac{R_{x2}.10}{R_{x2}+10}=7,5\)
\(\Leftrightarrow10R_{x2}=7,5R_{x2}+75\Leftrightarrow R_{x2}=30\left(\Omega\right)\)
\(\Leftrightarrow R_x=R_{x2}-R_2=30-25=5\left(\Omega\right)\)
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