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nono1-Visitor
1-Visitor
May 4, 2018
Solved

Solving equations

  • May 4, 2018
  • 2 replies
  • 4309 views

Hello, I'm Mathcad newbie and I can't get it to do anything useful...

Given the following set of equations:

eq2.png

I provide values for f and Cs:

eq3.png

Now I'd like to

1/ Provide Rp value and get Rs value

OR

2/ Provide Rs value and get Rp value.

 

How can it be done without duplicating the equations ?

 

Best answer by Fred_Kohlhepp

You are correct, the solve block will find a solution, but not the one you expect.  If you look at the grayed-out section where we solve for Rs, there are two answers.  If I turn that into a function, we get two real answers:

Capture.PNGAs with all math, "some experience required."

2 replies

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-MFra-21-Topaz II
21-Topaz II
May 4, 2018

Hi nono,

fornono.jpg

N
nono1-VisitorAuthor
1-Visitor
May 7, 2018

Thanks -MF- for taking time to answer.

However, your (or should I say Mathcad) solution is wrong : if you compute Rp "normally" with Rs = 100ohm you'll get Rp=2.6kOhm, not 1.6kOhm

This is scarying me about using such tool...

 

W
Werner_E25-Diamond I
25-Diamond I
May 7, 2018
@nono wrote:

Thanks -MF- for taking time to answer.

However, your (or should I say Mathcad) solution is wrong : if you compute Rp "normally" with Rs = 100ohm you'll get Rp=2.6kOhm, not 1.6kOhm

This is scarying me about using such tool...

 

No reason to be scared. If you look closely you will see that in FM's attempt, both intial values of Rp AND Rs change. FM had set Q=3 for reasons unknown to me.

 

In my answer above I forgot to square Q.

Here's the correct equation and we get the results you expected and also both results for R.s if we supply R.p:
B.png

W
Werner_E25-Diamond I
25-Diamond I
May 6, 2018

If you can live with the red errors from the numeric processor you could do it that way

B2.png

Otherwise you have to turn Q and equ into functions

N
nono1-VisitorAuthor
1-Visitor
May 7, 2018

I tried writing the exact same in a blank worksheet and the first solve output is "No solution found".

Untitled.png

Thanks for your time... I give up with Mathcad.

 

 

W
Werner_E25-Diamond I
25-Diamond I
May 7, 2018
@nono wrote:

I tried writing the exact same in a blank worksheet and the first solve output is "No solution found".

Untitled.png

Thanks for your time... I give up with Mathcad.

Chances are you have defined R.p above and assigned it a value.

Try the command clear.sym(R.p) before attempting to symbolically solve.

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