The formula used to multiply two complex numbers a+bi and c+di is:
Complex Numbers are explained in detail on the Wikipedia Complex Numbers Page.
The Excel Improduct function calculates the product of two or more complex numbers.
The syntax of the function is:
where the inumber arguments are between 2 and 255 Complex Numbers that you want to find the product of.
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 Improduct function as either:
The spreadsheets below show three examples of the Excel Improduct function:
Formulas:

Results:

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