Inverting Op Amp Homework - Solve V(output) Equation

[Lonesong]

Homework Statement

http://www.freeimagehosting.net/jqqoz

Homework Equations

V(output) = V(input)(R2/R1)

The Attempt at a Solution

The equation above is valid if the back part of the upper circuit (involving R3,R4 and the Earth wire) does not exist. I tried to improvise with it by using the equation V(output) = V(input)(R2+1/(1/R3+1/R4))/R1 however the answer is wrong. Just need some guidance as to what I am doing wrong.

 

[skeptic2]

Can you get the voltage at the inverting (-) input of the opamp?

 

[Lonesong]

That would be Vi/30 000= 3/30000 = 0.0001

edit: Oh wait, that's the current. Sorry.

 

[skeptic2]

Good, that's correct. So what is the voltage then?

 

[cepheid]

If you assume the things that you need to about ideal op-amps, then you basically just have a simple circuit to solve with one voltage source and a few resistors. I find it helpful to apply KCL at the node where R2, R4, and R3 all join. You can relate the voltage at this node to the input voltage, because you know what the voltage at the inverting input must be.

 

[skeptic2]

You have 100 uA flowing into the junction of R1, R2 and the op amp - terminal. Where is that current flowing to from there?