Worked example - design an inverting amplifier with gain -10. Choose R1 = 10 kohm (input resistor). Step 1: gain = -Rf/R1 = -10, so Rf = 100 kohm. Step 2: apply Vin = 0.5 V: Vout = -10 x 0.5 = -5 V. Step 3: sanity checks - input current = 0.5/10k = 50 uA flows through Rf (rule 2); V- sits at virtual ground ~0 V (rule 1); with +-15 V supplies the -5 V output is safely below saturation (~+-13 V). If Vin rose to 2 V, the ideal -20 V would clip at -13 V - a reminder that real op-amps saturate.