You need to use the IM...() functions to perform the computations in the quadratic formula in order to perform the computations in complex space. IM...() functions are listed in the "engineering functions" : https://support.office.com/en-us/art...__toc309306710
For example, after computing the discriminant (b^2-4ac), you would use the IMSQRT(b^2-4*a*c) function to get the imaginary square root of the discriminant. You could then use the IMDIV() and IMSUM() functions to complete the quadratic formula.
Note that complex numbers are displayed as text strings (a+bi or a+bj, depending on your preference) in Excel to the numbers full precision (which means they tend to have a lot of digits displayed). If you want to separate the real and imaginary parts of the complex numbers so that you can round them to something reasonable, you can use the REAL() and IMAGINARY() functions to separate these out.
I discussed this briefly in this "math science and engineering" discussion group, including a sample spreadsheet: http://www.excelforum.com/groups/mat...lex-roots.html I don't know how well this forum's "groups" area works as far as storing and downloading spreadsheets. At one time, it seemed to work.
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