Circuits Op Amp R equilvalent help

[Innuendo]

[SOLVED] Circuits Op Amp R equilvalent help!

Homework Statement

I'm working on Ideal op amps and I need to find the R equivalent of the system.
I'm not sure if this term is used in other places but R equivalent = total Resistance of the circuit.
http://img169.imageshack.us/img169/4386/reqrk6.png

2. The attempt at a solution
I have to place a voltage at the circle at the top left and use the voltage to divide the current to get the Req, but I don't know how to find the current. I think I have to use a current divider at the 10 and 40 k ohms but I can't seem to get the answer of 14 k ohms as the answer in the back of the book. Please help!

3. Relevant equations
I'm also confused if this is the Negative Resistance mentioned in the book. But the given equation for Req = -(R1/R2)R

 

[kamerling]

First calculate the voltage on the positive input of the opamp. That is just a simple voltage divider.
The opamp is used in a circuit with only negative feedback. What does this mean about the voltage on the negative input?

Once you know the voltage on the inputs of the opamp its simple to compute the input current of the circuit.

 

[Innuendo]

which resistance on the bottom is the one I need? I'm having problem on setting up that voltage divider at the positive input..
is it 30/70 * v ?

 

[kamerling]

which resistance on the bottom is the one I need? I'm having problem on setting up that voltage divider at the positive input..
is it 30/70 * v ?

yes. you just have a current going from the input through the 40k then the 30k and then to ground. If the resistance of the input of an op-amp isn't given, you can assume it is infinite.