 The Laws of Circuit - you can learn and practice by just reading

Circuits involved with Op Amps. In the analysis of a circuit with Op Amps, the nodal analysis (i.e., node voltage method) is a sure fire tool. The reason is like this: The two things you have to know and carry all the way to your excellent-pay job are that (1) the voltages at the inverting input (read as node n) and the voltage at the non-inverting input (read as node p) are the same, and (2) there are no currents flowing at the inverting and the non-inverting inputs toward the Op Amp. I wonder you missed one important thing above. We said the 'voltage at node n' or 'voltage at node p', as we expressed Vn and Vp, respectively. So now you know why the node voltage method is the method for circuits with Op Amps. This leads to another discussion. Vn is then the voltage between n and GND. Vp is the voltage between p and GND. Similarly, Vo, at the output is the voltage between o and GND. When one end of a source is connected to the GND, it's easy to figure the voltage at n or p, but when a source is not connected between a node and GND, you have to give a little more thought to find out the voltage at n or p.