(1/R1 + 1/R2 + 1/R4) * V1 - V2/R2 = Us/R1 + Is1(1/R2 + 1/R3) * V2 - V1/R2 = Is2 - Is1http://wenku.baidu.com/view/1a26938d8762caaedd33d4bc.html