Derivating problems
08-22-2018, 05:53 PM
Post: #1
 kjpuma Junior Member Posts: 1 Joined: Aug 2018
Derivating problems
Hi world, I'm starting using this hp prime v2. But, I've got a problem with this when trying to get partial derivatives or just one variable-derivative.
For example: when I type "diff(x^2-6*xy^2+y^2+10*y+18,X)", it just shows: 2x, taking Y as 0, what I want is to know what is wrong with my calculator or some settings I've chosen before.
08-22-2018, 09:46 PM
Post: #2
 Han Senior Member Posts: 1,882 Joined: Dec 2013
RE: Derivating problems
(08-22-2018 05:53 PM)kjpuma Wrote:  Hi world, I'm starting using this hp prime v2. But, I've got a problem with this when trying to get partial derivatives or just one variable-derivative.
For example: when I type "diff(x^2-6*xy^2+y^2+10*y+18,X)", it just shows: 2x, taking Y as 0, what I want is to know what is wrong with my calculator or some settings I've chosen before.

There are several problems with your command.

1. xy is different from x*y -- always be explicit with your operators as no software will be able to tell the difference between the mathematical product $$xy$$ from the variable name $$xy$$ without explicit operators.

2. You used a mixture of 'x' and 'X' for your variables. They are necessarily different. Moreover, the 'X' variable is a system variable whose content is always a real number (which may cause issues with symbolic manipulation).

3. Make sure you are using the CAS view and make sure that nothing is stored in the variables you are using. In your case, if you stick to using 'x' and 'y' (lowercase) then you can "delete" them using: purge([x,y]) in the CAS view.

Graph 3D | QPI | SolveSys
08-23-2018, 08:39 AM
Post: #3
 Aries Member Posts: 159 Joined: Oct 2014
RE: Derivating problems
(08-22-2018 05:53 PM)kjpuma Wrote:  Hi world, I'm starting using this hp prime v2. But, I've got a problem with this when trying to get partial derivatives or just one variable-derivative.
For example: when I type "diff(x^2-6*xy^2+y^2+10*y+18,X)", it just shows: 2x, taking Y as 0, what I want is to know what is wrong with my calculator or some settings I've chosen before.

image share online

Best,

Aries
08-23-2018, 09:17 AM
Post: #4
 Didier Lachieze Senior Member Posts: 1,649 Joined: Dec 2013
RE: Derivating problems
Frankly Aries, I don’t understand why you keep posting (big) TI nSpire screen shots in Prime threads. It doesn’t help the OP to solve his issue and doesn’t provide any added value.
It would have been much more useful if you have provided some explanation about how to work with the Prime or posted a Prime screen shot such as this one:

09-21-2018, 10:12 PM
Post: #5
 wheazel Junior Member Posts: 3 Joined: Sep 2018
RE: Derivating problems
Hello Community,
well, I am running into the same problem, and after research on the net, I found an old thread here in the forum that looked promising...but I ran into trouble and ended up in this thread.
What I've been looking for is to get the second derivative of a function. Sadly, I can't get it done.
As I understand, the only way to do this on the Prime is, to enter:
diff(f(x),$(x,2)) or, what seems the same: diff(f(x),x$2). Both Versions return a 0, which is not correct.
diff(f(x),x) works fine, so does f', f'' and so on.
I made sure, that I'm in CAS mode, and that the variables x is not in use.
btw..
f(x):=1/x^5

Thank's in advance for any suggestions, corrections or help
09-22-2018, 10:18 AM
Post: #6
 roadrunner Senior Member Posts: 449 Joined: Jun 2015
RE: Derivating problems
@ Aires
do you perhaps also post screenshots of the Prime on Texas Instruments website?

@wheazel

09-22-2018, 10:32 AM
Post: #7
 wheazel Junior Member Posts: 3 Joined: Sep 2018
RE: Derivating problems
so, as I got it now, diff works with expressions only, not with functions?

I got quite confused, because of the old thread that stated: diff(f(x),x$2 which is to me a function. 09-22-2018, 10:33 AM Post: #8  ggauny@live.fr Senior Member Posts: 580 Joined: Nov 2014 RE: Derivating problems of course not, this man don't know Tiplanet web site !!! Peccato. Gérard. 09-22-2018, 02:35 PM Post: #9  CyberAngel Member Posts: 299 Joined: Jul 2018 RE: Derivating problems (08-23-2018 09:17 AM)Didier Lachieze Wrote: Frankly Aries, I don’t understand why you keep posting (big) TI nSpire screen shots in Prime threads. It doesn’t help the OP to solve his issue and doesn’t provide any added value. It would have been much more useful if you have provided some explanation about how to work with the Prime or posted a Prime screen shot such as this one: I think that the moderators are NOT doing their job 09-22-2018, 05:02 PM Post: #10  Carsen Member Posts: 206 Joined: Jan 2017 RE: Derivating problems Hi Wheazel: (09-22-2018 10:32 AM)wheazel Wrote: Thank you roadrunner, so, as I got it now, diff works with expressions only, not with functions? diff() should work with expressions and functions. In other words, it should work regardless if the user stores it in 'f' or 'f(x)'. (09-21-2018 10:12 PM)wheazel Wrote: diff(f(x),$(x,2)) or, what seems the same: diff(f(x),x$2). Both Versions return a 0, which is not correct. diff(f(x),x) works fine, so does f', f'' and so on. diff(f(x),x), f', and f'' are all some of the correct ways to differentiate the expression/function. May I ask why you are using diff(f(x),$(x,2)) or diff(f(x),x$2)? I've never seen this syntax for a HP Prime ever. The '$' is very confusing.

Did you use the [HELP] key on the HP Prime in the upper left part of the keyboard? To use the [HELP] key on diff(), select diff() from the Calculus menu in the CAS portion of the toolbox and put it on the Entry Line. Then simply press the [HELP] key. You can see the syntax, a description, and examples. Check it out. It should clear up syntax issues, especially with the examples.

Let me know how everything works out!
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