An earlier post presented various methods for finding the roots of polynomial functions based on the use of linear interpolation.  It is sometimes advantageous to use a quadratic interpolation function, and methods using this approach will be presented in this and following posts.  The earlier functions and the new quadratic interpolation functions have been incorporated in the spreadsheet ItSolve Functions.xls. 

Any three points on a plane (other than co-linear points) will define a unique quadratic curve of the form ax² + bx + c = 0.  The User Defined Function (UDF) FITQUAD returns an array of the three coefficients, a, b and c: 

 A = ((Y2 – Y1) * (X1 – X3) + (Y3 – Y1) * (X2 – X1)) / ((X1 – X3) * (X2 ^ 2 – X1 ^ 2) + (X2 – X1) * (X3 ^ 2 – X1 ^ 2))
B = ((Y2 – Y1) – A * (X2 ^ 2 – X1 ^ 2)) / (X2 – X1)
C = Y1 – A * X1 ^ 2 – B * X1 

This quadratic equation may then be solved in the usual way to find the x values for which it evaluates to zero.  The procedure for applying this method to evaluating the roots of higher order functions is: 

• Select three known points on the function to be solved, in the region of the solution of interest, with at least one point on each side of the x-axis
• Fit a quadratic curve to these three points
• Find the root of the quadratic curve in the range of interest
• Find the y value of the higher order function at this x value.
• Replace the furthest outlying of the three trial points with this new point.
• Repeat until the error is acceptably small.

The screenshot below shows this procedure carried out on the spreadsheet, using the FitQuad function: 

Click to view full size

The process is automated using the UDF QuadSolve(): 

Inverse Quadratic Interpolation with QuadSolve