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05-28-2019
06:37 AM

05-28-2019
06:37 AM

Hi someone who can help solve this can't make it work....

Solved! Go to Solution.

1 ACCEPTED SOLUTION

Accepted Solutions

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05-28-2019
06:58 AM

05-28-2019
06:58 AM

You can't use "find" multiple times, but you can use multiple arguments in the find function and assign the result to a vector of variables:

14 REPLIES 14

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You can't use "find" multiple times, but you can use multiple arguments in the find function and assign the result to a vector of variables:

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05-28-2019
06:58 AM

05-28-2019
06:58 AM

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05-28-2019
07:40 AM

05-28-2019
07:40 AM

Re: 3 equations with 3 unknowns

Thank you !

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05-28-2019
05:04 PM

05-28-2019
05:04 PM

Re: 3 equations with 3 unknowns

Note that you are missing a few solutions...

Due to the guess values the solve block gave you just the first of each of four solutions.

With the expressions, it's easy to see what happens, e.g. if theta1 is varied:

Success!

Luc

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05-29-2019
01:38 AM

05-29-2019
01:38 AM

Re: 3 equations with 3 unknowns

thanks but so complicated it should not be. It's simple physics calculation

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05-30-2019
08:18 AM

05-30-2019
08:18 AM

Re: 3 equations with 3 unknowns

Hi again there are problems again with only 2 unknowns now.

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05-30-2019
06:09 PM

05-30-2019
06:09 PM

Re: 3 equations with 3 unknowns

1) You forgot about the guess values

2) Your equal sign in the fist equation is a definition, but it should be a boolean equal

3) While playing around with your sheet I experienced strange results and the reason was Primes infamous auto labelling. You have to take care and look that all variables (especially m1' and m2') are labelled the same (either all "variable" or all automatic "-").

4) Its no problem in your sheet but using the apostrophe the way you did is at least a bit problematic as you may confuse it with the prime symbol of a derivation.

Worksheet in format P5 attached

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05-31-2019
02:53 AM

05-31-2019
02:53 AM

Re: 3 equations with 3 unknowns

Thank you for your answer but I still get change values. I've done everything your said...

I send my document again.

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05-31-2019
04:05 AM

05-31-2019
04:05 AM

Re: 3 equations with 3 unknowns

As Werner explained: this autolabellling feature of Prime has good and bad sides.

Notice that some of your m1 variables are labelled 'variable' and others are labelled '(variable)'. I'm afraid that makes them different. You should set the '(variable)' labels explicitly to 'variable'.

How about:

Oh, and by the way, given the units of your m' variables, they appear to be mass per unit of time quantities. So you probably chose the right notation, with the prime ('). Note however that prime notation is also used in solve blocks to solve differential equations using the odesolve() function instead of find().

Success!

Luc

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05-31-2019
05:57 AM

05-31-2019
05:57 AM

Re: 3 equations with 3 unknowns

I don't think you understand me I can make it work the other way. But can't make it work with solve blocks. Thanks.

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05-31-2019
06:49 AM

05-31-2019
06:49 AM

Re: 3 equations with 3 unknowns

@kenan2 wrote:

Thank you for your answer but I still get change values. I've done everything your said...

I send my document again.

Look at item 2) in my answer above. You still have the wrong equal sign in your first equation

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06-01-2019
01:40 PM

06-01-2019
01:40 PM

Re: 3 equations with 3 unknowns

`No I have not. See the new one I had attached! :D`

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06-01-2019
01:43 PM

06-01-2019
01:43 PM

Re: 3 equations with 3 unknowns

I've done everything you said. It still gives strange values. Especially if I change where I guess then the answer changes. But may well see it works in your document but not in mine has also marked everything as variable

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06-01-2019
04:30 PM

06-01-2019
04:30 PM

Re: 3 equations with 3 unknowns

@kenan2 wrote:

I've done everything you said.

NO! You did NOT! You still did not change the equal sign in your first equation.

It should NOT read m3 **:**= m1 + m2 but rather should read m3 **=** m1 + m2 with the big fat comparison equal sign!

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06-01-2019
04:26 PM

06-01-2019
04:26 PM

Re: 3 equations with 3 unknowns

@kenan2 wrote:

`No I have not. See the new one I had attached! :D`

Sure your still have the wrong assignment equal sign (:=)!

!!!!Your new picture clearly shows that you still have the wrong equal sign!!!!

m3 := ... IS WRONG! You need the big fat boolean equal sign here as in your second equation. Look at the sheet I posted in my last answer.