Desolve gives wrong results
12-23-2019, 03:25 PM (This post was last modified: 12-23-2019 07:25 PM by COB160.)
Post: #1
 COB160 Junior Member Posts: 35 Joined: Jul 2017
Desolve gives wrong results
Goodafternoon everybody,
I am dealing with desolve HP prime function, in particular if I type desolve(y'=-x/y,y) i get two wrong results, wich are: sqrt(-2*G_0-x^2 ) and -sqrt(-2*G_0-X^2). In fact the correct result should be, according to wolframalpha: sqrt(-x^2+2c) and -sqrt(-x^2+2c). Does the difference depend on the interpretation of G_0 or c constant? Otherwise the result is incorrect with respect of the sign.
12-23-2019, 06:47 PM
Post: #2
 CyberAngel Member Posts: 299 Joined: Jul 2018
RE: Desolve gives wrong results
(12-23-2019 03:25 PM)COB160 Wrote:  Goodafternoon everybody,
I am dealing with desolve HP prime function, in particular if I type desolve(y'=-x/y,y) i get two wrong results, wich are: sqrt(-2*G_0-x^2 ) and -sqrt(-2*G_0-X^2). In fact the correct result should be, according to wolframalpha: sqrt(-x^2+c) and -sqrt(-x^2+c). Does the difference depend on the interpretation of G_0 or c constant? Otherwise the result is incorrect with respect of the sign.

The constant(s) could be anything
thus the answers are all correct.
12-23-2019, 07:28 PM
Post: #3
 COB160 Junior Member Posts: 35 Joined: Jul 2017
RE: Desolve gives wrong results
(12-23-2019 06:47 PM)CyberAngel Wrote:
(12-23-2019 03:25 PM)COB160 Wrote:  Goodafternoon everybody,
I am dealing with desolve HP prime function, in particular if I type desolve(y'=-x/y,y) i get two wrong results, wich are: sqrt(-2*G_0-x^2 ) and -sqrt(-2*G_0-X^2). In fact the correct result should be, according to wolframalpha: sqrt(-x^2+c) and -sqrt(-x^2+c). Does the difference depend on the interpretation of G_0 or c constant? Otherwise the result is incorrect with respect of the sign.

The constant(s) could be anything
thus the answers are all correct.

Yes, but for what reason the calculator changes the conventional way to express the variable in such simple differential equations? Is there any way to get the conventional result?
12-24-2019, 03:18 AM
Post: #4
 Tim Wessman Senior Member Posts: 2,293 Joined: Dec 2013
RE: Desolve gives wrong results
Yes, you look at it and say "brain, that is a c".

The reason why is because c is used for other variables in the system and would be treated as such if you returned it in such a way in subsequent calculation.

TW

Although I work for HP, the views and opinions I post here are my own.
12-24-2019, 06:21 AM
Post: #5
 Stevetuc Senior Member Posts: 313 Joined: Jan 2014
RE: Desolve gives wrong results
(12-23-2019 06:47 PM)CyberAngel Wrote:
(12-23-2019 03:25 PM)COB160 Wrote:  Goodafternoon everybody,
I am dealing with desolve HP prime function, in particular if I type desolve(y'=-x/y,y) i get two wrong results, wich are: sqrt(-2*G_0-x^2 ) and -sqrt(-2*G_0-X^2). In fact the correct result should be, according to wolframalpha: sqrt(-x^2+c) and -sqrt(-x^2+c). Does the difference depend on the interpretation of G_0 or c constant? Otherwise the result is incorrect with respect of the sign.

The constant(s) could be anything
thus the answers are all correct.

Correct but unfriendly, as the constant could be simplified to c_1 say, even if c is reserved.
Its fortunate indeed that the OP didnt require the answer to
desolve(y'=-i*x/y,y)
Wolfram gives this as
y(x) = -sqrt(c_1 - i*x^2) and y(x) = sqrt(c_1 - i*x^2)

Prime with Simplify:Maximum gives the undeniably spectacular
Code:
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[.....]

I had to truncate the result as its longer than the forum can handle!
Code:
 Please correct the following errors before continuing: The message is too long. Please enter a message shorter than 65535 characters (currently 133051).
12-24-2019, 08:17 AM
Post: #6
 COB160 Junior Member Posts: 35 Joined: Jul 2017
RE: Desolve gives wrong results
(12-24-2019 03:18 AM)Tim Wessman Wrote:  Yes, you look at it and say "brain, that is a c".

The reason why is because c is used for other variables in the system and would be treated as such if you returned it in such a way in subsequent calculation.

Very very funny answer! I can’t stop laughing.

The variable c could be any number but there is no reason to arbitrarily chance the sign of it.
12-24-2019, 08:25 AM
Post: #7
 COB160 Junior Member Posts: 35 Joined: Jul 2017
RE: Desolve gives wrong results
Is there any other way to came out with a symbolic solution of differential equations? Maybe using different function or changing calculator’s Settings
12-24-2019, 03:44 PM
Post: #8
 toml_12953 Senior Member Posts: 2,129 Joined: Dec 2013
RE: Desolve gives wrong results
(12-24-2019 06:21 AM)Stevetuc Wrote:
(12-23-2019 06:47 PM)CyberAngel Wrote:  The constant(s) could be anything
thus the answers are all correct.

Correct but unfriendly, as the constant could be simplified to c_1 say, even if c is reserved.
Its fortunate indeed that the OP didnt require the answer to
desolve(y'=-i*x/y,y)
Wolfram gives this as
y(x) = -sqrt(c_1 - i*x^2) and y(x) = sqrt(c_1 - i*x^2)

I had to truncate the result as its longer than the forum can handle!

Are you sure about that 317th character? It looks wrong to me.

Tom L
Cui bono?
12-24-2019, 03:53 PM (This post was last modified: 12-24-2019 03:58 PM by Stevetuc.)
Post: #9
 Stevetuc Senior Member Posts: 313 Joined: Jan 2014
RE: Desolve gives wrong results
(12-24-2019 03:44 PM)toml_12953 Wrote:
(12-24-2019 06:21 AM)Stevetuc Wrote:  Correct but unfriendly, as the constant could be simplified to c_1 say, even if c is reserved.
Its fortunate indeed that the OP didnt require the answer to
desolve(y'=-i*x/y,y)
Wolfram gives this as
y(x) = -sqrt(c_1 - i*x^2) and y(x) = sqrt(c_1 - i*x^2)

I had to truncate the result as its longer than the forum can handle!

Are you sure about that 317th character? It looks wrong to me.
Could be a translation error, its probably fine..maybe view the answer on the Prime?
There may be errors in the bits that couldn't be pasted though, I haven't checked through it yet..
12-24-2019, 04:50 PM
Post: #10
 toml_12953 Senior Member Posts: 2,129 Joined: Dec 2013
RE: Desolve gives wrong results
(12-24-2019 03:53 PM)Stevetuc Wrote:
(12-24-2019 03:44 PM)toml_12953 Wrote:  Are you sure about that 317th character? It looks wrong to me.
Could be a translation error, its probably fine..maybe view the answer on the Prime?
There may be errors in the bits that couldn't be pasted though, I haven't checked through it yet..

I was just hoping someone would waste time counting characters!

Tom L
Cui bono?
12-24-2019, 04:55 PM
Post: #11
 Stevetuc Senior Member Posts: 313 Joined: Jan 2014
RE: Desolve gives wrong results
(12-24-2019 04:50 PM)toml_12953 Wrote:
(12-24-2019 03:53 PM)Stevetuc Wrote:  Could be a translation error, its probably fine..maybe view the answer on the Prime?
There may be errors in the bits that couldn't be pasted though, I haven't checked through it yet..

I was just hoping someone would waste time counting characters!

Yeah, would'nt it be nice if the prime had a search for character function for answers like that one and jokes like that one!
12-24-2019, 06:54 PM
Post: #12
 parisse Senior Member Posts: 1,315 Joined: Dec 2013
RE: Desolve gives wrong results
I can not reproduce.
desolve(y'=-i*x/y,y) on the Prime returns [sqrt(-i*x^2-G0),-sqrt(-i*x^2-G0)]. simplify leaves this answer unchanged.
12-24-2019, 09:43 PM (This post was last modified: 12-24-2019 09:48 PM by Stevetuc.)
Post: #13
 Stevetuc Senior Member Posts: 313 Joined: Jan 2014
RE: Desolve gives wrong results
The result I reported was on android 2.1.14181 (2018 10 16)
On android 2.1.14346 (2019 11 21) it simply crashes after much text output to console.

12-25-2019, 09:13 AM
Post: #14
 Aries Member Posts: 159 Joined: Oct 2014
RE: Desolve gives wrong results
(12-24-2019 08:25 AM)COB160 Wrote:  Is there any other way to came out with a symbolic solution of differential equations? Maybe using different function or changing calculator’s Settings

For comparison's sake, let's see how CX II's like:

(y=-sqrt(c-x^2) et (x^2)-c<=0) V (y=sqrt(c-x^2) et (x^2)-c<=0).

Have a pleasant XMas,
Aries
12-25-2019, 02:15 PM (This post was last modified: 12-25-2019 02:26 PM by Marcel.)
Post: #15
 Marcel Member Posts: 182 Joined: Mar 2014
RE: Desolve gives wrong results
Hi!
The first post from COB160 change FROM:

… to wolframalpha: sqrt(-x^2+c) and -sqrt(-x^2+c).

TO

… to wolframalpha: sqrt(-x^2+2c) and -sqrt(-x^2+2c).

If you look the first response from CyberAngel you will see the original post.

Question, Why doing this?
Is it to adjust WolframAlpha to the response of the Prime..
This is not a good way to make a champion.
Marcel
12-25-2019, 04:39 PM
Post: #16
 parisse Senior Member Posts: 1,315 Joined: Dec 2013
RE: Desolve gives wrong results
(12-24-2019 09:43 PM)Stevetuc Wrote:  The result I reported was on android 2.1.14181 (2018 10 16)
On android 2.1.14346 (2019 11 21) it simply crashes after much text output to console.
I checked with the emulator (old Mac 2018 07 06) and with the devel windows version, both return the expected answer and simplify does nothing. If something wrong happens elsewhere, I can not do anything to fix it myself.
12-25-2019, 05:18 PM
Post: #17
 CyberAngel Member Posts: 299 Joined: Jul 2018
RE: Desolve gives wrong results
(12-25-2019 04:39 PM)parisse Wrote:
(12-24-2019 09:43 PM)Stevetuc Wrote:  The result I reported was on android 2.1.14181 (2018 10 16)
On android 2.1.14346 (2019 11 21) it simply crashes after much text output to console.
I checked with the emulator (old Mac 2018 07 06) and with the devel windows version, both return the expected answer and simplify does nothing. If something wrong happens elsewhere, I can not do anything to fix it myself.
12-25-2019, 05:29 PM
Post: #18
 lrdheat Senior Member Posts: 857 Joined: Feb 2014
RE: Desolve gives wrong results
On my G2, if I type in the expression in question, the unit displays [-x/y’] as it’s solution. If I use a desolve example, and edit the letters to show our example, I get the correct answer, [sqrt(-x^2*G_0 -sqrt(-x^2-2*G_0]. How do I correctly enter this so to not have to bring up the example, and edit it to show my intended expression?
12-25-2019, 06:25 PM
Post: #19
 Stevetuc Senior Member Posts: 313 Joined: Jan 2014
RE: Desolve gives wrong results
(12-25-2019 04:39 PM)parisse Wrote:
(12-24-2019 09:43 PM)Stevetuc Wrote:  The result I reported was on android 2.1.14181 (2018 10 16)
On android 2.1.14346 (2019 11 21) it simply crashes after much text output to console.
I checked with the emulator (old Mac 2018 07 06) and with the devel windows version, both return the expected answer and simplify does nothing. If something wrong happens elsewhere, I can not do anything to fix it myself.

Hopefully Tim or Cyrille can take a look.
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