Following a comment here: Solving cubic and quartic equations with Excel the Quartic and CubicC User Defined Functions (UDFs) have been amended to fix a problem occurring with some combinations of coefficients.
The updated spreadsheet (including full open source code) can be downloaded from: Polynomial.zip
As well as functions to solve Quartic, Cubic and Quadratic equations analytically the spreadsheet includes an iterative method that will solve polynomial functions of any order, using the Jenkins-Traub method, and a function to evaluate polynomial functions of any order.
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