The Excel IMARGUMENT function returns the argument θ, (an angle expressed in radians), of a supplied complex number.
θ is the angle that satisfies the following equation:
The syntax of the Imargument function is:
where the inumber argument is a complex number.
In the spreadsheet below, the Excel Imargument function is used find the angle θ, of five different complex numbers.
Formulas:
 Results:

Note that, in the above example spreadsheet:
Further details of the Excel Imargument function are provided on the Microsoft Office website.
If you get an error from the Excel Imargument Function, this is likely to be one of the following:
#NUM!    Occurs if the supplied inumber argument is not recognised as a complex number. 
#VALUE!    Occurs if the supplied inumber argument is a logical value. 