how can I use parameter instead of equation in dsolve

Hi

I want to solve an ordinary differential symbolic equation with dsolve. But I want to save my equation in a parameter and use this parameter in dsolve. My codes are coming below:

      clear all
      syms a x
      l=a*x;
      dsolve('Dx=l')

these codes don't work. What should I do?

Thanks a lot

Answer 1

[Kai Gehrs]

Hi Frank,

you can try something like

clear all; syms a x; l=a*x; dsolve(['Dx= ' char(l)])

The point is that variables/parameters inside a string cannot be "evaluated". But as the example suggests you can use the command CHAR to convert a symbolic expression to a character string and use the brackets to concatenate strings appropriately.

Hope this helps and best regards,

-- Kai

[frank]

It works.

Thank you so much

Answer 2

[Walter Roberson]

dsolve( subs(sym('Dx=l')) )

[Andrei Bobrov]

Hi Walter!

This is for MATLAB R2012a only?

[Walter Roberson]

Shouldn't be. It is just using a straight-forwards subs().

[frank]

When I use this command, MATLAB returns the error:

Warning: Empty equation

> In dsolve at 109

In chechdsolve2 at 3

ans =

[ empty sym ]

Thanks

[Luan Trinh]

Hi everyone,

I want to know how to solve some ODEs like this: clear all; syms a x; l=a*x; dsolve(['D2x*' char(l)+ 'Dx*x' == 0])

Could you help me?