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fsolve problem

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Akshayy Garg
Akshayy Garg on 30 Jun 2011
Answered: Walter Roberson on 16 Mar 2019
Hi I am having two problems: I--> I used fsolve in two ways. one by declaring the function inside fsolve and second by making a function. Both give different answers. Second method gives complex solution which is not possible. II--> For some equations fsolve is complex answers. Can i by some method limit fsolve to give real solution.

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Answers (2)

Jesús Zambrano
Jesús Zambrano on 16 Mar 2019
Edited: Jesús Zambrano on 16 Mar 2019
You can try with the function solve.
For example, to force getting real solutions:
syms x
eqn = x^5 == 3125;
solve(eqn, x) % gives all the solutions
solve(eqn, x, 'Real', true) % for ce to give the real solutions

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madhan ravi
madhan ravi on 16 Mar 2019
but solve() is used in the answer
Jesús Zambrano
Jesús Zambrano on 16 Mar 2019
I meant: function solve.

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Walter Roberson
Walter Roberson on 16 Mar 2019
It is not possible to set bounds for fsolve() . You can set the FunValCheck option for fsolve() to tell it to check each iteration for nan and inf and complex values, but that will cause fsolve() to error, not to somehow avoid or work around the problem.
If you just happened to be working with a single function with a single variable, then fzero() permits a range to be passed for x0.
If you have the symbolic toolbox, then vpasolve() permits you to pass bounds for multiple variables.

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