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

[

attachment=1916]

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.

No response yet. Still I'd like to know.

- Is it a bug?

- Am I wrong to expect a proper derivative plot?

(04-28-2015 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.

Hello,

probably the problem is a result of the fact, that the ABS-Function leads to a not continously differntiable function. At the roots of the basic function the ABS-function 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

For the moment, you'll need to use that SLOPE command as shown in the other thread. It *was* an issue. :-)

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.

(04-28-2015 03:58 PM)StephanP Wrote: [ -> ]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.

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.