Prev: surface
Next: Bezout matrix
From: Alan Weiss on 22 Dec 2009 15:51
If I am not mistaken, the expression sqrt(abs(f)) appears in both the
numerator and denominator of the second line, and nowhere else.
Therefore, it cancels. Therefore, you cannot solve the equation as
written for f, since F does not depend on f. Perhaps you made a typo?

Alan Weiss
MATLAB mathematical toolbox documentation

Catherin wrote:
> I need to solve the following function:
>
>>> syms ks f d rey
>
>>> F=(-4)*log10((ks/d)/3.71+...
> 2.51/(2*sqrt(abs(2*f))*rey))*sqrt(abs(f))-1;
>
>>> f=solve(F,f)
>
> But the answer is:
>
> Warning: Explicit solution could not be found.
>> In solve at 170
>
> f =
> [ empty sym ]
>
> Does some one know how can I solve this symbolic equation?, I don't want
> to use fsolve because I need to use it several times in my program.
>
> Thanks.
From: Catherin on 22 Dec 2009 16:22
Hi Alan,

It is not a typo, the original equation is:

1/sqrt(f)=-4*log10((ks/d)/3.71+2.51/(2*sqrt(2*f)*rey))

"f" can't be cancelled because one of "f" is inside a logarithm.

thanks,

catherin

Alan Weiss <aweiss(a)mathworks.com> wrote in message <hgrbfq$bp6$1(a)fred.mathworks.com>...
> If I am not mistaken, the expression sqrt(abs(f)) appears in both the
> numerator and denominator of the second line, and nowhere else.
> Therefore, it cancels. Therefore, you cannot solve the equation as
> written for f, since F does not depend on f. Perhaps you made a typo?
>
> Alan Weiss
> MATLAB mathematical toolbox documentation
>
> Catherin wrote:
> > I need to solve the following function:
> >
> >>> syms ks f d rey
> >
> >>> F=(-4)*log10((ks/d)/3.71+...
> > 2.51/(2*sqrt(abs(2*f))*rey))*sqrt(abs(f))-1;
> >
> >>> f=solve(F,f)
> >
> > But the answer is:
> >
> > Warning: Explicit solution could not be found.
> >> In solve at 170
> >
> > f =
> > [ empty sym ]
> >
> > Does some one know how can I solve this symbolic equation?, I don't want
> > to use fsolve because I need to use it several times in my program.
> >
> > Thanks.
 | 
Pages: 1
Prev: surface
Next: Bezout matrix