MATLAB: How to use implicit model functions in Curve fitting toolbox

cftoolcurve fittingCurve Fitting ToolboximplicitMATLAB

I am trying to fit some data on a custom implicit equation (2nd degree polynomial in x and y; see below) and obtain the coefficients (a,b and c) for that data.
But the cftool does not have an implicit option in its custom equation tab.
Any help is appreciated on the topic. I understand that people are coding it in some ways and i would really appriciate if a GUI based answer is also possible.

Best Answer

You can consider the equation like this
where . Then define x, y, and z in MATLAB code like this
x; y; % your data pounts
z = zeros(size(x));
and then use cftool() with x, y, and z variables. And in custom equation, write
a*x^2+b*y^2+c*y
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