怎么求这个的等效电阻

i = i1R2/R1 + i1 = ( 1 + R2/R1 )i1 = ( R1 + R2 )i1/R1；u = 2i1 + i1R2 = ( 2 + R2 )i1Ri = u/i = ( 2 + R2 )R1/( R1 + R2 ) = 2R1/( R1 + R2 ) + R1//R2即计算等效电阻时，受控源相当一个电阻，阻值为受控源电势除以流过的电流。R = 2i1/i = 2i1/[( R1 + R2 )i1/R1 ] =2R1/( R1 + R2 )；所以列出i =( R1 + R2 )i1/R1后，可直接写出Ri = 2i1/i + R1//R2 =2R1/( R1 + R2 ) + R1R2/( R1 + R2 ) 。