solve vs. fsolve

Question

byungkeuk cho 2020-5-27

0
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/534803-solve-vs-fsolve

回答： Walter Roberson 2020-5-27

采纳的回答： Star Strider

在 MATLAB Online 中打开

I am trying to solve non linear equations with solve or fsolve.

But I found difficulties in using them.

for example,

i would like to solve the equation below.

a^2 = 4

then, i can get the answer with the script below.

syms a;
eqn1 = a^2 == 4;
[a1] = solve(eqn1);

then, it gives me a = -2, 2

but when I used fsolve, it gives me only one value according to initial point.

this is the script.

fun = @test;
x0 = [1];
[a2] = fsolve(fun,x0)
function F = test(x)
F(1) = -x(1)^2 + 4;
end

if I set x0 as [-1], then it gives me another value but still only one value.

How can I get the all values with fsolve?

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Answer 1

Star Strider 2020-5-27

0
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/534803-solve-vs-fsolve#answer_439708

在 MATLAB Online 中打开

Give fsolve different starting points, one positive and one negative:

eqn1 = @(x) x.^2 - 4;
for k = 1:2
    a2(k) = fsolve(eqn1, 5*(-1)^k);
end
a2

producing:

a2 =
           -2            2

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byungkeuk cho 2020-5-27

Thank you for your quick response.

But if I have no idea on "how many staring points needed" or "what points they should be" due to complicated equations, then how can I solve it?

Star Strider 2020-5-27

My pleasure.

The easiest way would be to plot it, at least to find the real roots. If some or all of the roots are complex, this becomes more difficult, however fsolve will take complex initial estimates and will use them to return complex roots. In that situation, it will be necessary to experiment.

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