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StuartGregory1-Visitor
1-Visitor
April 11, 2010
Question

Another Simple Question!

  • April 11, 2010
  • 3 replies
  • 3733 views
Hi

Hopefully the last daft question!

Attached is my formula so far.

Q1: Why so many decimal places in the 'Find' answer? I've checked and the display is set to three decimal place and I can't find any other place to set the display to less than the 18 shown!

Q2: I'd like to plot Va and Vb against f. But all the graph examples show simple formulas put directly into the y placeholder. Two problems - that doesn't work and how do I split the results into graphs showing Va and Vb? How can I use the results of the 'Find' calculation in a graph? Should I be using something else? I would like to start with just the amplitude of the complex number but want to display the complex representations later.

Cheers
James

3 replies

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ptc-13682881-Visitor
1-Visitor
April 11, 2010
On 4/11/2010 2:50:34 PM, stuartgregory wrote:
>Hi
>
...
...
>Q2: I'd like to plot Va and Vb
>against f. ...

>Cheers
>James
>_______________________________

Maybe like this ?

jmG



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AlvaroDíaz12-Amethyst
12-Amethyst
April 12, 2010
Just another two ways, in separate files for more easy comparing. Notice that the numeric solution is slowest, but if no closed form is found by find is the only one avaible.

Regards.
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Al20001-Visitor
1-Visitor
April 11, 2010
For defining Va(f) and Vb(f) I just copied/pasted the results of Find. Maybe another collab shows you how to do that in a better way.

Hope this helps. Saludos,

Al
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Al20001-Visitor
1-Visitor
April 11, 2010
Ups! Sorry, I forgot to delete the picture.
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TomGutman1-Visitor
1-Visitor
April 11, 2010
The reason for all those decimal places is that you chose to use symbolic, rather than numeric, evaluation. The rules for symbolic evaluation a quite different from those for numeric evaluation, and just about everything in the documentation is based on numeric evalulation.

As for plotting, you have nothing to plot. You are just calculating a pair of complex numbers. To get a meaningfule plot you need a function, or multiple data points.
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� � � � Tom Gutman
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StuartGregory1-VisitorAuthor
1-Visitor
April 11, 2010
Thanks for the reply:

Is there a setting to suppress the decimal places when using symbolic evaluation?

Yes, I only have one value of Va and Vb - I can't see how I can use Mathcad to vary f over a range and thereby have different values of Va and Vb to plot?

I can see how I can put f into a place holder on a 2D plot but not how I can squash the whole equation in the y placeholder.

James
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TomGutman1-Visitor
1-Visitor
April 11, 2010
The first question, is why use symboic evaluation in the first place? Mathcad is essentially a numeric calculator, with some symbolic capabilities tacked on. Generally, you should only use symblic evaluation when you have a definite and compelling reason to do so. Symbolic evaluation is not a replacement for, nor the equivalent of, numeric evaluation.

You can use the float keyword to control the number of places for symbolic evaluation. But note that float controls the wholw evaluation, not just the display. Using a small value of float will result in roundoff errors in the result (how much depending the the extent of the calculations), not just change the display value. You can also use the format results dialog, checking the option to apply it to symbolic results. That has its own quirks.

To be able to vary f you must make f define your results as functions with formal parameter f. You can then evaluate the function for any desired values of f.
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