-7+(i1-i3)+2(i1+i2)=0,7=3i1+2i2-i3.....................(1) -12+3(i2+i3)+2(i1+i2)=0,12=2i1+5i2+3i3...........(2) -9+(i3-i1)+4i3+3(i2+i3)=0,9=-i1+3i2+8i3.............(3) (1)x3 +(2) 33=11i1+11i2,i1+i2=3 而 i1+i2=i=3A。Vb=3x2=6v,Va已知=3v,Uab=3-6=-3v。