Có : R1nt R2 \(\Rightarrow R_{12}=R_1+R_2=4+8=12\Omega\)
R3nt R4\(\Rightarrow R_{34}=R_3+R_4=6+6=12\Omega\)
Có : R3nt R4\(\Rightarrow I_3=I_4=I_{34}=0,2A\)
\(\Rightarrow U_{34}=I_{34}.R_{34}=0,2.12=2,4\left(V\right)\)
Mặt khác : (R1nt R2)//(R3nt R4)
\(\Rightarrow U_{12}=U_{34}=U_{1234}=2,4V\)
\(\Rightarrow I_{12}=\frac{U_{12}}{R_{12}}=\frac{2,4}{12}=0,2\Omega\)
Có : ((R1nt R2)//(R3nt R4))nt R5
\(\Rightarrow I_{AB}=I_5=I_{12}+I_{34}=0,2+0,2=0,4\left(A\right)\)
Có : (R12//R34)nt R5
\(\Rightarrow U_{AB}=U_{1234}+U_5\)
\(\Rightarrow U_5=U_{AB}-U_{1234}=40-2,4=37,6\left(V\right)\)
Khi đó :
R5=\(\frac{U_5}{I_5}=\frac{37,6}{0,4}=94\Omega\)
