断s1s2闭s3，相当于就接了个r1，所以总电压是20*0.5=10v；断s2闭s1s3，相当于r1r2并联，0.9*r2*20/(r2+20)=10,得出r2=25欧姆；断s1s3闭s2，相当于r1r2串联，10/(20+25)=0.22A,0.22*20=0.44V
