Plot derivative of an ABS( function ) fails (RESOLVED)

04272015, 12:29 PM
(This post was last modified: 04282015 04:07 PM by StephanP.)
Post: #1




Plot derivative of an ABS( function ) fails (RESOLVED)
While testing the HP Prime I ran into the following odd behaviour.
Symb F1(X)=X^2 10X +21 (between absolute brackets) F2(X)=dF1(X)/dX=X (the derivative of F1) Plot F1 plots fine, bouncing nicely at x=3 and x=7 F2 does not plot well. It gives three horizontal line segments. Y=1 for x < 3 Y=1 for 3 < x < 7 Y=1 for x > 7 If I use the function Slope in the plot windows, then correct slope values are given for F1. For F2, slope values are "NaN". When I remove the absolute brackets from F1, F2 plots perfectly. 

04282015, 12:25 PM
Post: #2




RE: Plot derivative of an ABS( function ) fails
No response yet. Still I'd like to know.
 Is it a bug?  Am I wrong to expect a proper derivative plot? 

04282015, 01:28 PM
Post: #3




RE: Plot derivative of an ABS( function ) fails
(04282015 12:25 PM)StephanP Wrote: No response yet. It has been less than a full day since your initial post. Please be patient for a reply, many users do not check in hourly or even daily. It seems your interest is more curiosity than some urgent need, so please continue exploring it yourself in parallel; you may find an answer before another user gets to reply. Bob Prosperi 

04282015, 01:58 PM
Post: #4




RE: Plot derivative of an ABS( function ) fails
Hello,
probably the problem is a result of the fact, that the ABSFunction leads to a not continously differntiable function. At the roots of the basic function the ABSfunction creates a discontinuity! What is the slope at x = 3 or x = 7? I think the value of the derivative at this points represented by NaN (not a number). regards Wolfgang 

04282015, 02:19 PM
(This post was last modified: 04282015 02:25 PM by CR Haeger.)
Post: #5




RE: Plot derivative of an ABS( function ) fails  
04282015, 02:27 PM
(This post was last modified: 04282015 03:03 PM by Tim Wessman.)
Post: #6




RE: Plot derivative of an ABS( function ) fails
For the moment, you'll need to use that SLOPE command as shown in the other thread. It *was* an issue. :)
TW Although I work for HP, the views and opinions I post here are my own. 

04282015, 03:58 PM
(This post was last modified: 04282015 04:03 PM by StephanP.)
Post: #7




RE: Plot derivative of an ABS( function ) fails
Yes, SLOPE(F1,X) works.
I did find that other thread, but as I was experiencing crashes using slope() (lowercase from the Catalog), I went for the d/dx method. This seemed to work fine, apart from the ABS() case. Still it's a bit confusing, these variants of slope() and SLOPE(). I had to type it manually. Thanks CR, Tim. 

04282015, 04:45 PM
Post: #8




RE: Plot derivative of an ABS( function ) fails (RESOLVED)
TW Although I work for HP, the views and opinions I post here are my own. 

04282015, 04:54 PM
Post: #9




RE: Plot derivative of an ABS( function ) fails (RESOLVED)
(04282015 03:58 PM)StephanP Wrote: Yes, SLOPE(F1,X) works. Toolbox > App > Function > SLOPE is what I use while in the Function App. Another interesting way to key this into CAS and send to Function App: f(x):= X^2 10X +21 F1:=f F2:=f' which defines f(x), F1 as f(X) and F2 as f'(X). Hit Plot and it should display ok. 

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