Problem Calculating Nonlinear Indefinite Integral

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Dimitrios Kampitsis 2019-11-30

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https://ww2.mathworks.cn/matlabcentral/answers/494024-problem-calculating-nonlinear-indefinite-integral

评论： Dimitrios Kampitsis 2019-12-4

Hello,

I am trying to calculate a nonlinear integral t^5.2 *exp((-x^6.2-x*(0.2*4.5^6.2))/4.5^6.2) in order to solve an equation, where the unknown is the lower limit of integrations.Although i use "int" in order to calculate the integral, Matlab returns the equation.Is my equation too complicated or there is another way to get a solution ?I am quite new in Matlab, so any help would be appreciated.

I would like to thank you in advance for the time dealing with my query.

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Dimitrios Kampitsis 2019-12-1

My goal is to solve the following equation where the unknown is the limit of integration (k), where a and b are constants

I tried only with int in order to solve the integral and afterwards, if i get a solution from my integration, i am planning to try and solve the equation. However i am not sure if this is the right way to do it.

syms x
g = x^5 *exp((-x^5-x*(4^6))/4^6) ;
int(g,x,k,k+2)

I havent tried yet trapz. Moreover i didnt try integral because i am looking for a symbolic solution.

Thanks again.

darova 2019-12-1

You should use fsolve and integral

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Answer 1

David Wilson 2019-12-2

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https://ww2.mathworks.cn/matlabcentral/answers/494024-problem-calculating-nonlinear-indefinite-integral#answer_404221

Since you neglected to tell us values for a and b, I'll just set them to a=2 and b=1. You have also two definitions for g(x), so I've used the one in your code. Like the previous poster suggested, I'll embed integral inside fsolve:

g = @(x) x.^5.*exp((-x.^5-x*(4^6))/4^6) ;
Q = @(k) integral(g,k,k+2);
a = 2; b = 1; % Set these to whatever
k0 = 1; % Need a start guess here. 
k = fsolve(@(k) Q(k) - a/b, k0)

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Dimitrios Kampitsis 2019-12-4

Thank you for your help, it works and i get a solution. I have tried it also with a more complex equation and it works fine. I really appreciate your help.

However for a specific value of my constants fsolve returns a value (Equation solved) that it is not a root of my equation. I confirmed that using another software "Mathcad".

Could this be a problem of function tolerance ?

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