# How to find value of X when Y reaches maximum value

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RAHUL YADAV on 18 Aug 2020
Commented: RAHUL YADAV on 20 Aug 2020
Hello,
I am new to MATLAB Simulink, I want output as the value of X when Y reaches maximum.
Y is a calculated value from an equation and is a function of X Show 1 older comment
RAHUL YADAV on 18 Aug 2020
This model is in Simulink so it would be better if the answer is related to same..
But still I can use user defined function to use MATLAB code having X and Y axes as input...
Rik on 18 Aug 2020
I don't know much about Simulink, so I can't help you there. Once you have a function in Matlab itself you can use fminsearch. You want the maximum, so you would need to invert your function (inverted=@(varargin) -YourFunction(varargin{:});)
RAHUL YADAV on 19 Aug 2020
problem solved using Leonardo's approach

Leonardo Maia on 18 Aug 2020 Not sure what kind of output you want, but one way would be to get the derivative of your y(x) function. When it's "zero", it's at a maximum or minimum. I added quotes to "zero" because it's not going to be zero due to the time step of the solver. In the above example I'm getting the absolute of the derivative and when it's <= 0.01, I get the x value.

Show 1 older comment
RAHUL YADAV on 19 Aug 2020
1) I got the zero crossing of y=f(x) (maxima)... 2) And after using the switch I am getting a spike at which the condition is approx zero 3)At last, what can I do get the output of value of X... like in this case ''displays 0.13.. something''
Leonardo Maia on 19 Aug 2020
RAHUL YADAV on 20 Aug 2020
Thanks

Sara Boznik on 18 Aug 2020

#### 1 Comment

RAHUL YADAV on 19 Aug 2020
Thanks..