Failure in initial objective function evaluation. LSQNONLIN cannot continue

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so i'm supposed to do nonlinear regression to find the values of a,b and the equation y given the value of x small x=2.6 .i tried using lsqnonlin but i think i'm doing it wrong ,i don't have anything related to nonlinear regression in my course pdf's and this is the first time i'm attempting lsqnonlin . Additionally i have values of x and the result of the equation y , %X=[0.5 1 2 3 4 ]; %Y=[10.4 5.8 3.3 2.4 2 ]; as an example ,don't have to do anything with them ,and the equation is :
and what i attepmpted is :
%% calculus

Accepted Answer

Walter Roberson
Walter Roberson on 6 Dec 2020
lsqnonlin() is going to pass the given function a single vector of values that is the same length as the second parameter to lsqnonlin(), which is a scalar in your code.
Thus, your y is going to be invoked as y(2.6) so the 2.6 is going to be positionally matched to a and a will be valid inside the function body. However, your y actively uses its second and third parameters, b and x so the function will fail.
y = @(ab) ((ab(1)+sqrt(x))./(ab(2).*sqrt(x))).^2
and pass in an initial vector of length 2

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