http://www.postimg.com/86000/circuit-85584.jpg

I understand that Vout for an op amp is A(V+ - V-) but I cannot understand why for the right circuit the equation is phrased thus. Where does R1+R2 / R1 come from?

 

What is the original circuit?

 

Oh gosh, my bad.

http://www.postimg.com/image/86000/qn2-85615.jpg

 

The first thing to make sure that you agree with is the circuit itself. Are you comfortable that the redrawn circuit represents the case if V2 is set to 0V? If so, then the next step is to write V+ in terms of V1. Now write V- in terms of Vout. Now apply the approximation that V+ is equal to V- for a properly operating opamp circuit and you have it.

 

Thank you!