The Excel Imsqrt function returns the square root of a supplied complex number.
The syntax of the function is:
Where the inumber argument is the complex number that you want the square root of.
In the spreadsheets below, the Excel Imsqrt function is used to calculate the square root of five different complex numbers.
Formulas:

Results:

Note that, in the above example spreadsheet, the integer 6, used in cell B4, is equal to the complex number 6+0i.
More details and examples of the Excel Imsqrt function are provided on the Microsoft Office website.
If you get an error from the Excel Imsqrt 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. 