a,Ia=0A=>mạch điện cân bằng\(=>\dfrac{R1}{R3}=\dfrac{R2}{R4}=>\dfrac{15}{45}=\dfrac{30}{R4}=>R4=90\Omega\)
b,=>(R1 // R2)nt(R3 // R4)
\(=>Im=I13=I24=\dfrac{U}{RTd}=\dfrac{75}{\dfrac{R1R3}{R1+R3}+\dfrac{R2R4}{R2+R4}}=4A\)
\(=>U13=I13R13=45V=U1=>I1=\dfrac{U1}{R1}=3A\)
\(=>U24=I24R24=30V=U2=>I2=\dfrac{U2}{R2}=1A=>I1=Ia+I2=>Ia=I1-I2=2A\)
c,=>(R1 nt R2)//(R3 nt R4)
\(=>U12=U34=75V=>I12=\dfrac{U12}{R12}=\dfrac{5}{3}A=I1=>U1=I1R1=25V\)
\(=>I34=\dfrac{U34}{R34}=1A=I3=>U3=I3R3=45V\)
\(=>Uv=-U1+U3=45-25=20V\)
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