R1=2 ôm nhé:
Ta có: R2//R1 nên:
\(U_{12}=U_1=R_1\cdot I_1=2\cdot1,5=3\left(V\right)\Rightarrow U_3=U-U_{12}=10-3=7\left(V\right)\\ Rtđ_{12}=\dfrac{R_1\cdot R_2}{R_1+R_2}=\dfrac{2\cdot12}{2+12}=\dfrac{12}{7}\left(\Omega\right)\\ \Rightarrow I_3=I_{12}=\dfrac{U_{12}}{Rtđ_{12}}=\dfrac{3}{\dfrac{12}{7}}=\dfrac{17}{4}\left(A\right)\\ \Rightarrow R_3=\dfrac{U_3}{I_3}=\dfrac{7}{\dfrac{7}{4}}=4\left(\Omega\right)\)
Vậy R3=4 ôm
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