This feedback-forces-equal-inputs lesson treats negative feedback as an ideal op-amp contract: while the output stays inside the rails, the two input pins are driven to the same voltage. The arithmetic is exact only for the stated 0 V input-node row and ideal high-gain feedback model. Real op-amps have finite gain, input offset, bandwidth limits, slew rate, rail headroom, tolerances, finite precision, and measurement uncertainty.
An ideal op-amp in negative feedback is modeled as an exact equal-input contract. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
Negative feedback checks equal input voltages
In the linear ideal model, feedback drives the two input pins to 0 volts together.