To convert Fahrenheit to Celsius, we use the formula: °C = (°F - 32) x 5/9. For 98.6° F, we subtract 32 to get 66.6, then multiply by 5/9 to arrive at 37.0°C. For 100° F, the calculation is (100 - 32) x 5/9 = 37.8°C. Finally, for 212° F, we calculate (212 - 32) x 5/9 = 100.0°C. This formula is essential for converting temperatures between these two scales.