1. The problem statement, all variables and given/known data Find range of resistance R4 and voltage U4 so that the range of current I is [tex]I \in [-3A,-1A][/tex] E=5V, E4=6V, Ig=1A, Ig4= -3A, R1=2ohm, R2=3ohm, R3=5ohm 2. Relevant equations -Kirhoff's law -Potential of nodes method 3. The attempt at a solution Firstly, I set current I= -3A. Setting potential V3=0 V, current I23=0.25 A. Using Kirchhoff's law on node 2 ,I12=1.25 A. On node 1 ,I14=1.75 V. Now potential of nodes are V1=5 V, V2=1.25 V, V3=0 V V4=1.5 V. With potential of nodes method on node 4, R4=2.09 ohm. Then, U4= -4.5 V. In my book's solution, it says that R4 starts from 0.22 ohm and U4 starts from -4.5 V. Could someone check this?