# -40 (number)

 Cardinal -40 Binary -101000 or 11011000in two's complementin a signed byte Hexadecimal -28, or D8in two's complementin a signed byte

Minus forty is:

• the temperature (in degrees) at which Celsius and Fahrenheit scales read the same; often referred to as 40 below.

This can be shown easily. To convert from Fahrenheit to Celsius, one subtracts 32, then multiples by 5/9. This is expressed mathematically as C = (5/9)(F − 32), where F = Fahrenheit and C = Celsius. Alternatively F = C(9/5) + 32 .

Therefore, to find this special temperature, choose a formula and set the variables equal, denoting a temperature that is the same in both scales:

      F = F(9/5) + 32
(−4/5)F = 32
F = −40


or

     C = (5/9)(C − 32)
(9/5)C = C − 32
(4/5)C = −32
C = −40


These are mathematically equivalent demonstrations.

Also, the Fahrenheit and Celsius temperature conversion formulae can be represented in the particularly nice forms (using -40)

[itex] F = \frac{9(C - (-40))}{5} + (-40)\,[itex]

and

[itex] C = \frac{5(F - (-40))}{9} + (-40)\,[itex]

which may be easier for mental calculation.

sl:-40 (število)

• Art and Cultures
• Countries of the World (http://www.academickids.com/encyclopedia/index.php/Countries)
• Space and Astronomy