As pointed out by Mitschele (1), the application of the classical formulas for conversions between Celsius (C) and Fahrenheit (F) temperature scales has been a source of confusion for students and professionals alike. I am surprised that those who teach temperature conversion formulas have not introduced the easier-to-retain relationship based on the fact that –40 F° is the same temperature as –40 C°. This method derives from the symmetrical relationship between the Celsius and Fahrenheit temperature scales: (F + 40) = 9/5 (C + 40). This relationship is easily obtained from the classic relationship, F + (9/5) C + 32, by adding 40 to each side of the equation, and then factoring 9/5 on the right side. The conversion is a three-step procedure, regardless of the direction of conversion:
Add 40 to the seed number, either F or C
Multiply by the appropriate slope, either 9/5 or 5/9
Subtract 40 from the product to obtain the final result
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