4.(2)求R3b a a 292 42 49 2 29 2g2 492 29 2g2 b 49 (a)开路:R3b=2/(4+2)/(2+1)=1g2 (b)短路:Rab=2/(4/2+2/1)=1g24. (2) 求 Rab . 2 1 2 b a 2 4 4 (a) 开路：Rab=2//(4+2)//(2+1)=1 (b) 短路：Rab=2//(4//2+2//1)=1 a 2 4 1 2 4 2 b