How to solve an equation in a certan interval.

Question

Nuno 2012-12-2

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/55446-how-to-solve-an-equation-in-a-certan-interval

Problem:

syms w; % variable, that define the y axis
lam = 4*pi()^2 % constant, that define the x axis
tan(w/2) == w/2 - (4/(lam))*(w/2)^3 % equation

That equation with that constant has many solutions. What I pretend is to resolve the equation (maybe with solve) for lam = 4*pi()^2, in a certain interval of y, in order to catch a specific solution. For example, I know that for that constant there is a solution of w/pi() = 2 in the interval w/pi()=[1.5 2.5], (y axis).

I was trying with this:

syms w;
lam=4.*pi().^2;
x = fsolve(@(w) tan(w/2) == w/2 - (4./(lam)).*(w/2).^3, [1.5*pi() 2.5*pi()]);
X=x/pi()

But is giving me some problems that I can´t resolve.

On Maple software the structure of the program is more or less the same, but it gives me the solutions that I want in specific intervals of y axis.

Matlab has to do it too or it doesn't?

Thanks to all.

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Answer 1

Walter Roberson 2012-12-2

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The function you provide to fsolve() needs to have a numeric result rather than a logical result. Convert your form A==B to (A)-(B) or (B)-(A)

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Nuno 2012-12-8

In "fsolve" can you use an interval? because if you put a number in the place of the interval, the fsolve gives you one number, but if you put [number1 number2], it gives you two numbers. It is ridiculuos when you should get just one, the solution.

Walter Roberson 2012-12-8

No, fsolve() does not take an interval. When you provide multiple values for x0 it becomes an equation with multiple variables. fzero() is the one that takes an interval.

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