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SOLVED
Level 6

## Issues solving a second order ODE

Hi folks,

I am trying to solve a second order differential equation in Mathcad 15 to model the diffusion of a substrate in a spherical pellet. Regrettably, Odesolve gives me the following message: 'This function cannot be used here' (see attached file). I have tried to rearrange the equation in different forms to see if that helped, but I still have the same problem. Do you know how can I solve the issue?

Best,

Daniela

1 ACCEPTED SOLUTION

Accepted Solutions

## Re: Issues solving a second order ODE

This indeed solves the issue. Thanks Alvaro!

Best,

Daniela

5 REPLIES 5

## Re: Issues solving a second order ODE

Did you set up the Odesolve block correctly? For example:

- Wrote the derivatives using the proper symbols and operators?

- Is the highest derivative linear (not multiplied with t or the solution function y(t))?

- Did you provide sufficient/enough initial conditions?

Success!
Luc

## Re: Issues solving a second order ODE

- I used prime notation for the ODE and boundary conditions in addition to using the boolean equal. I think there should be no problems with this point.

- Yes, the highest derivative is linear

- Yes, it's a second order ODE and I specified 2 boundary conditions.

I attached the file in case you want to have a look at it

Best,

Dan

## Re: Issues solving a second order ODE

No file attached!

Highlighted

## Re: Issues solving a second order ODE

Hi. You have 2/r*cs'(r) as part of the differential equation, and set c(0)=0. (The other condiont isn't initial, it's boundary, and have not problems) Mathcad can't handle r=0 here. Or change to c(0.000001)=0 or change the numerator where r appears as r+0.0000001.

Best regards.

Alvaro.

## Re: Issues solving a second order ODE

This indeed solves the issue. Thanks Alvaro!

Best,

Daniela