This input-current-is-zero lesson separates the ideal op-amp input pin from the resistor-current ledger around it. The exact rows assume zero input-pin current, so the stated 1 A resistor current must balance through the surrounding feedback path instead. Real op-amps draw bias currents and have leakage, offset, finite input impedance, temperature drift, tolerances, finite precision, and measurement uncertainty.
Ideal op-amp inputs do not consume current; resistor currents balance elsewhere. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
The input pins take zero current
The resistor current is 1 ampere, but the ideal input-pin current is zero.
I+=I−=0A
Resistor current balances resistor current
The 1 ampere that enters through one resistor leaves through feedback.