ODE error

[PW_10297783]

Hello,

I am using Adams to solve the differential equations as below but failed, may anyone could suggest how to solve it or should I use another equation.The error can only disappear when x was deleted in the equation.

.

,x from 0 to 10000

Thanks in advance.

PW

[Werner_E]

You have to write y(x), not just y.

Furthermore I guess you will run into problems because the right hand term is not defined for the start value x=0 (division by zero).

I changed the start value to x=10^-10 and it works OK.

Here done in Mathcad 15 using "odesolve" with Adams/BDF chosen as algorithm.
I let x only run up to 1000 - you can see that y approaches zero continually.

For further questions please attach your worksheet, not just a picture. And in case you are using Prime also state which version.

[PW_10297783]

Thank you Werner,

Please fine my error enclosed.I am using Prime 7.0.

Thank for guiding me how to proceed.

PW

[Werner_E · Accepted solution]

Your sheet is still missing - its not attached.

I see that you changed the initial value for x significantly now from 0 to 3600!

In Prime you would have to use the commands inside of a solve block!

The syntax for odesolve had changed, too, from legacy Mathcad to Prime (look it up in the help):

The attached file is in Format of Prime 6., so you should be able to open it with P7.

[PW_10297783]

thank you,I drag the sheet one more time,

[LucMeekes]

Should be more like in attached.

Notice how Prime handles solve blocks different from Mathcad (15 and before).

Success!
Luc

[PW_10297783]

Dears,

I come back to this topic further more,How could I make the If program fuction in this Odesolve modle please.I use Prime 9.0 now.

[LucMeekes]

Not sure what you want. Note that in your solve block, y is a function: y(x), described by the ODE, and dependent on c. Now it appears that you want c (that y depends on) to change depending on the value of y....?

I see a chicken<>egg problem.

Success!
Luc

[Werner_E]

I, too, see the chicken and egg problem mentioned by Luc 😉

I parametrized your solve block so you can create a solution for any given value of c.

Then I defined the two solution functions for c=1 and c=4 (y1 and y5).

Finally a function y was defined which equals y1 as long as its value is over 0.05 and then drops down to y5.
But I hardly can imagine that this is really what you are looking for ...

Prime 6 file attached

[Werner_E]

Here is a different approach with a similar result.

I used the if-function but of course you could as well use the program if-statement.

Still not sure if you are really looking for something like this.

Prime 6 file attached

[PW_10297783]

This is exactly what I need,thank you both.

[Werner_E]

Had not seen your second posting with the file and in the meantime edited my answer above and added a file created in P6 (I never bothered to install P7 or P8).

Basically the same as what was posted by Luc - use a solve block and change the syntax of "odesolve".

BTW, Prime also allows units in a solve block with "odesolve" (legacy Mathcad didn't), so you should take advantage of Primes unit-awareness and use units, too.

[LucMeekes]

The differential equation:

Has solution:

Now with:

we know c also.

So:

We can compare the numeric solution

with the exact solution:

The difference is:

Success!
Luc

[Werner_E]

Forgot to mention that in case you are using a current version of Prime, the ODE can also be solved symbolically

It also looks like we could add an initial condition

But the first attempt using a numeric value failed

Using 1/10 instead of 0.1 does the trick:

Using the prime symbol for the derivative you may even omit the function argument x, but I won't suggest doing so

[PW_10297783]

May you upload your file for symbolical solution in your free time please,I am not able to proceed with the mistake below

Thanks

[LucMeekes]

Prime 7 does not support solving differential equations symbolically. You have to upgrade to Prime 9 for that.

Success!

Luc

[PW_10297783]

Thanks for your rapid reply Luc,

Also for your proffessional response for solve block/numeric solution last month.

[Werner_E]

You don't show the sheet and you don't say what the error message is you get, so we have to guess and either you forgot to clear y (at least for symbolical usage) before you solve symbolically (you can't solve for a variable or function which already is defined)

or its because you are still using Prime 7.  Version 9 is the first one with the ability to solve ODEs symbolically. That's the reason why I wrote  "Forgot to mention that in case you are using a current version of Prime, the ODE can also be solved symbolically".

For whatever its worth I nonetheless attach the P9 sheet with the symbolic solutions.