**Part Number:**LM318

Recently, I was looking at the calculation of the input impedance of the op-amp

This article describes in detail how to calculate the input impedance of the inverting amplifier circuit, not just the RIN，

But how to understand v+ and v-, in the article , the v- is vtest, v+=itest*Ra, I do not know how to get this formula, look forward to the expert's detailed answer.