R12=R1*R2/(R1+R2)=1*2/(1+2)=0.67 Ом
R56=R5*R6/(R5+R6)=5*6/(5+6)=2.73 Ом
R12356=R12+R3+R56=0.67+3+2.73=6.4 Ом
Ro=R12356*R4/(R12356+R4)=6.4*4/(6.4+4)=2.46 Ом
Io=U/Ro=10/2.46=4.07 A
I4=U/Ro=10/4=2.5 A
I3=Io-I4=4.07-2.5=1.57 A
U3=I3*R3=1.57*3=4.71 B
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