Hello Physics Forums community, I'm a new member and this is my first post. I have a circuits exam tomorrow, and though I grasp the majority of the material I'm having a hard time understanding the solution to a HW problem that I attached as a picture. Problem: Show that the circuit has A=Vo/Vi= -k(R2/R1) where k= 1+ R4/R2 + R4/R3. One of the issues is I have no clue how to derive: V1= -(R2/R1)VI And also the KCL @ V1; I understand where -(V1/R2) comes from. and from where (V1-AV1)/R4 comes from. But I have no clue how the middle term, -V1/R1, is derived. Shouldn't it instead be -V1/R3?? Could this be a typo or am I just a n00b? If so, please help me out. Thanks very much.