# fzero of function 3 variables

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raffaele orlando on 9 Apr 2020
Edited: Walter Roberson on 9 Apr 2020
function dydt = eqdiff(t,y,lambda)
dydt=-lambda*y
lambda=1
I write
fzero( @eqdiff(t,y,lambda),2)
matlab give me errore message
how i can solve the zero of a function

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raffaele orlando on 9 Apr 2020
i want to use fzero for the function dydt . can i use fzero for function as dydt(3 variables)?if yes how?
Torsten on 9 Apr 2020
So in the simple case you stated, you want to know for which value of y the expression -lambda*y becomes zero ?
raffaele orlando on 9 Apr 2020
yes

Ameer Hamza on 9 Apr 2020
To solve the equation with multiple input variables, use fsolve. Also, the input can be multi-dimensional, but the variable needs to be the same. For example
fsolve(@(x) eqdiff(x(1),x(2),x(3)), zeros(1,3))
function dydt = eqdiff(t,y,lambda)
dydt=-lambda*y;
lambda=1;
end

Walter Roberson on 9 Apr 2020
fzero() is designed for single functions of one variables that return scalar values.
fsolve() from the Optimization toolbox can handle multiple variables (and multiple functions.)
Sometimes what you can get away with is
fminunc( @(tyl) eqdiff(tyl(1), tyl(2), tyl(3)).^2, initial_values)
However,
function dydt = eqdiff(t,y,lambda)
dydt=-lambda*y
lambda=1
That last line confuses me. You have lambda on input but you assign 1 to it inside the function? What are you expecting that would do for you?
I suspect that you are using the wrong approach to what you are doing. I suspect that you are trying to solve a boundary value problem; see https://www.mathworks.com/help/matlab/boundary-value-problems.html for those.
Your function has trivial solutions: just let y or lambda be 0.