有四个电阻分别为R1,R2,R3,R4电阻分别为4欧，2欧，2欧和402欧，R1和R2串联，R2又和R3串联，然后和R4并联,

设流过r2的电流为i2

u1=u2=i2r2=i2×3ω=3i2

u3=u-u2=18-3i2

i=i1+i2=1+i2

p3=u3i=(18-3i2)(1+i2)=36

18+18i2-3i2-3i2²=36

i2²-5i2+6=0

(i2-2)(i2-3)=0

①i2=2a

u1=u2=i2r2=2a×3ω=6v

r1=u1/i1=6v/1a=6ω

u3=u-u1=18v-6v=12v

i=i1+i2=1a+2a=3a

r3=u3/i=12v/3a=4ω

②i2=3a

u1=u2=i2r2=3a×3ω=9v

r1=u1/i1=9v/1a=9ω

u3=u-u1=18v-9v=9v

i=i1+i2=1a+3a=4a

r3=u3/i=9v/4a=2.25ω