The first formula is the square root of a ratio.
The second formula you posted is not quite right. The correct formula is Fn = 17.3 * SQRT(distance * (path_length - distance) / (Frequency_GHz * path_length))
The 17.3 comes out of converting the wavelength to a frequency (wavelength = speed of light / frequency) and taking the constant out of the square root. If you square 17.3 you get 299.29 which is close to the speed of light in meters per second, 299,792,458. The constant is divide by 1000 to get the answer in meters rather than km when the distances are in km. The constant should be 17.31451581766005 but it is pointless using so many decimal places, so it is rounded to 17.3
- - - Updated - - -
This link provides some background on microwave link calculations
http://www.piscespacific.org/livesite/files/Link_Budget_Calculation.pdf