I am trying to compute the zero of a function using fzero, but the system always says my function must be finite and real. How can I solve that? Thank you!

Question

Tianlan Yang 2021-9-24

评论： Tianlan Yang 2021-9-24

I want to find the zero of f(x) = 2(3x+4)(x^3+4x^2-10)/(x^2(3x+8)^2)

Here is the screenshot for the error:

Answer 1

Matt J 2021-9-24

编辑：Matt J 2021-9-24

Clearly, your function is non-finite at x=0. There is a factor of x^2 in its denominator.

Regardless, fzero() is the wrong tool for analyzing a rational function. Just use roots() to get the roots of the numerator polynomial.

r=[roots([1 0 4 -10]).'  ,-4/3];
r=real( r(  abs(imag(r))<1e-10 ))
r = 1×2
    1.5568   -1.3333

And, of course, you must make sure that none of the numerator roots coincide with any denominator roots, but that is obvious here by inspection.

Tianlan Yang 2021-9-24

So for this example, how can I apply the root function?