Within the world of complex numbers, the 'imaginary' value, i is used to represent the square root of 1.
A Complex Number is composed of a real number combined with an imaginary number.
E.g. The complex number, z, is written as
The Excel IMAGINARY function returns the imaginary coefficient of a supplied complex number
 i.e. for the complex number a + bi, the Excel Imaginary function returns the value b.
The syntax of the function is :
where the inumber argument is a complex number.
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 argument can be supplied to the Excel Imaginary function as any of the following:
  A simple number 
  A complex number encased in quotation marks  eg. "5+3i" 
  A reference to a cell containing a complex number or a numeric value 
  A value returned from another Excel function or formula 
These are all included in the examples below.
In the example below, the Excel Imaginary function is used find the imaginary coefficient of different complex numbers.
Formulas:

Results:

Further information on the Excel Imaginary function can be found on the Microsoft Office website.
If you get an error from the Excel Imaginary 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: 