When adding complex numbers, the real and imaginary coefficients are added separately.
I.e. the equation to find the sum of two complex numbers a+bi and c+di is:
Complex Numbers are explained in detail on the Wikipedia Complex Numbers Page.
The Excel Imsum function calculates the sum of two or more complex numbers.
The syntax of the function is:
where the inumber arguments are between 2 and 255 Complex Numbers.
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 Imsum function as either:
The spreadsheets below show three examples of the Excel Imsum function:
Formulas:

Results:

Note that, in the above example spreadsheet:
Further details of the Excel Imsum function are provided on the Microsoft Office website.
If you get an error from your Excel Imsum 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. 