- −40 (number)
Minus forty is the unique
temperature (in degrees) at which theCelsius andFahrenheit scales read the same; it is often referred to as 40 below, which is short for "40 degrees below zero"To convert from Fahrenheit to Celsius, one subtracts 32, then multiplies by 5/9. This is expressed mathematically as "C" = (5/9)("F" − 32), or "F" = "C"(9/5) + 32. It is easily checked that if "C" = −40 then "F" = −40 and "vice versa".
Using −40, the Fahrenheit and Celsius temperature conversion formulae can be represented by the following:: and: which may be easier for mental calculation.
To prove that F = C, when either are equal to -40,
Let's let and
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