Then I began watching the Dave's movie about op-amps -
youtu.be/7FYHt5XviKc but I didn't last until the end, I'll watch it some other time. What he tries so hard to explain (inverting amplifier) is very easy with a simple demonstration. Two variable voltage sources (the one is the input voltage source, the other is the op-amp output) of opposite polarity are connected to each other through a network of two resistors (R1 and R2) in series.