Feedback Current Matches Input Current

The output sign is the direction needed to balance current at the inverting node. 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 pin current is zero

The input pin takes zero current, so the node balance is resistor-in equals feedback-out.

I_{\text{pin}} = 0\ \text{A}

The checked current is 1 ampere in and 1 ampere back through feedback.

I_{\text{in}} = I_{\text{feedback}} = 1\ \text{A}