I1+I2-I3=0R1·I1+R3·I3=Us1R2·I2+R3·I3=Us2代入数据:I1+I2-I3=0 ①I1+6·I3=15 ②3·I2+6·I3=9 ③解方程组①②③I1=3(A),I2=-1(A),I3=2(A)