When dividing complex numbers a+bi by complex number c+di, the formula used is as follows:
Further information on Complex Numbers can be found on the Wikipedia Complex Numbers Page.
The Excel IMDIV function calculates the quotient of two complex numbers (ie. divides one complex number by another).
The format of the function is :
where the inumber arguments are both Complex Numbers, and you want to divide inumber1 by inumber2.
Note that complex numbers are simply stored as text in Excel. When a text string in the format "a+bi" or "a+bj" is supplied to one of Excel's builtin complex number functions, this is interpreted as a complex number.
Also the complex number functions can accept a simple numeric value, as this is equivalent to a complex number whose imaginary coefficient is equal to 0.
Therefore, the inumber arguments can be supplied to the Excel Imdiv function as either:
  A simple number  
or  
  A complex number encased in quotation marks  eg. "5+3i"  
or  
  A reference to a cell containing a complex number or a numeric value  
or  
  A value returned from another Excel function or formula 
These are all included in the examples below.
In the example below, the Excel Imdiv function is used find the difference between different pairs of complex numbers.
The format of the functions is shown in the spreadsheet on the left and the results are shown in the spreadsheet on the right.
Formulas:

Results:

Further information on the Excel Imdiv function can be found on the Microsoft Office website.
If you get an error from your Excel Imdiv function this is likely to be one of the following :
#NUM!    Occurs if the supplied inumber argument is not recognised as a complex number 
#NAME?   
Occurs when Analysis ToolPak addin is not enabled in your Excel. + Show How to Do This in Excel 2003: 