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# Fit operation with defined variables

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Isac Grossa on 1 Mar 2021
Commented: Isac Grossa on 2 Mar 2021
Hi there. I have the following example of code:
mydata_x = random_data % isn't important how data i have
mydata_y = random_data
a = 10;
b = 0.5;
c = 3;
To calculate the fit of mydata I use to calculate d:
[fit1,gof1] = fit(mydata_x,mydata_y,'(10-0.5.*x.*d)/(3+0.5.*x.*d)');
There is a way to substitute 10,0.5 and 3 with the equivalent vaiables: a,b and c; like the following code:
[fit1,gof1] = fit(mydata_x,mydata_y,'(a-b.*x.*d)/(c+b.*x.*d)');
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### Accepted Answer

Steven Lord on 1 Mar 2021
See the "Create Fit Options and Fit Type Before Fitting" example on the documentation page for the fit function. It uses the 'problem' option to specify that certain variables in the expression to be fitted are problem parameters that will be specified when the expression is fitted, not coefficients whose values should be determined by the fitting process.
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Isac Grossa on 2 Mar 2021
Thanks.

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