### 5.2.1 Notation of physical constants

If you want to use a physical constants inside Xcas, put it’s name between two characters `_` ("underscore"). Don’t confuse physical constants with symbolic constants, for example, e,π are symbolic constants as `_c_,_NA_` are physical constants.
Input :

`_c_`

Output speed of light in vacuum :

`299792458_m*s```^```-1`

Input :

`_NA_`

Output Avogadro’s number :

`6.0221367e23_gmol```^```-1`