a) R1//R2//R3
=>\(\dfrac{1}{R_{123}}=\dfrac{1}{R_1}+\dfrac{1}{R_2}+\dfrac{1}{R_3}=\dfrac{1}{30}+\dfrac{1}{30}+\dfrac{1}{\dfrac{30}{4}}=\dfrac{1}{5}\)
\(\Rightarrow R_{123}=5\Omega\)
R4ntR123
=> \(R_{td}=R_4+R_{123}=25+5=30\Omega\)
b) R4ntR123
=> \(I_4=I=I_{123}=\dfrac{45}{30}=1,5\left(A\right)\)
\(U_1=U_2=U_3=U_{AB}-U_4=45-1,5.25=7,5\)
\(I_1=\dfrac{U_1}{R_1}=\dfrac{7,5}{30}=0,25\left(A\right)\)
\(I_2=\dfrac{U_2}{R_2}=\dfrac{7,5}{30}=0,25\left(A\right)\)
\(I_3=\dfrac{U_3}{R_3}=\dfrac{7,5}{\dfrac{30}{4}}=1\left(A\right)\)
R3=30:4=7,5\(\Omega\)
ta có R4nt(R1//R2//R3)=> Rtđ=\(R4+\dfrac{R1.R2.R3}{\left(R1.R2\right)+\left(R2.R3\right)+\left(R3.R1\right)}\)=30\(\Omega\)
B) I=\(\dfrac{U}{Rt\text{đ}}=\dfrac{45}{30}=1,5A\)
Vì R4ntR123=> I4=I123=I=1,5A
Vì R1//R2//R3=>U1=U2=U3=U123=I123.R123=1,5.5=7,5V
=> I1=\(\dfrac{U1}{R1}=\dfrac{7,5}{30}=0,25A;I2=\dfrac{U2}{R2}=\dfrac{7,5}{30}=0,25A;I3=\dfrac{U3}{R3}=\dfrac{7,5}{7,5}=1A\)
