CAS: solve solutions
08-25-2016, 07:18 PM (This post was last modified: 08-26-2016 11:43 AM by compsystems.)
Post: #1
 compsystems Senior Member Posts: 1,165 Joined: Dec 2013
CAS: solve solutions
hp-prime
solve(((x+5)*e^x) = ((x+5)*e^(2*x))) [ENTER] returns ->
why the CAS returns only one answer?

Expanded form
expand( (x+5)*e^x) = (x+5)*e^(2*x) ) [ENTER] returns -> x*e^x+5*e^x = x*e^(2*x)+5*e^(2*x)

https://www.wolframalpha.com/input/?i=(x...*e%5E(2*x) [ENTER] returns ->
x = -5 // real solution
x = 0 // real solution
x = 2*π*i*n where n ∈ Z // a symbolic answer =]
08-25-2016, 08:22 PM
Post: #2
 Steve Keeley Junior Member Posts: 18 Joined: Apr 2014
RE: CAS: solutions
I tried it (with different parentheses) and, after a cryptic warning, got the same answer Mathematica did.
08-26-2016, 07:49 AM
Post: #3
 retoa Member Posts: 168 Joined: Jan 2015
RE: CAS: solve solutions
Hi Steve!

(08-25-2016 08:22 PM)Steve Keeley Wrote:  I tried it (with different parentheses) and, after a cryptic warning, got the same answer Mathematica did.

How did you write it to obtain the full answer?
I get the same result as compsystems
08-26-2016, 01:12 PM
Post: #4
 Dirk.nl Member Posts: 141 Joined: Apr 2015
RE: CAS: solve solutions
Have you select Complex? (CAS settings)
I have the same answer as Steve (10077)

— Dirk Hartland
08-26-2016, 01:17 PM
Post: #5
 compsystems Senior Member Posts: 1,165 Joined: Dec 2013
RE: CAS: solve solutions
but the answer (-5) is not complex output is real number
08-26-2016, 01:50 PM
Post: #6
 Dirk.nl Member Posts: 141 Joined: Apr 2015
RE: CAS: solve solutions
Have you try it?
Selected complex, answer is {-5 0}

— Dirk Hartland
08-26-2016, 03:20 PM
Post: #7
 Tim Wessman Senior Member Posts: 2,114 Joined: Dec 2013
RE: CAS: solve solutions
Specifying a range works here as well.

-10..10 returns -5,0 also

Perhaps bernard can shed some light as to why it only finds the one solution in this case without specifying a range.

TW

Although I work for the HP calculator group, the views and opinions I post here are my own.
08-26-2016, 10:07 PM
Post: #8
 Steve Keeley Junior Member Posts: 18 Joined: Apr 2014
RE: CAS: solve solutions
(08-26-2016 07:49 AM)retoa Wrote:  Hi Steve!

How did you write it to obtain the full answer?
I get the same result as compsystems

solve((x+5)*e^x = (x+5)*e^(2*x),x)

(note that on the calculator I didn't have the parentheses around the 2*x term since in textbook form it was obvious that the whole term was part of the power of e)

My settings were Exact on, Complex on, Use Sq on, Use i off, Principal off and increasing off.

The warning I got was:

Warning: solving x in equation
-x*(e^x)^2+x*e^x-5*(e^x)^2+5*e^x=0

Anybody know what this warning means?
08-26-2016, 10:37 PM
Post: #9
 retoa Member Posts: 168 Joined: Jan 2015
RE: CAS: solve solutions
Right, with complex on it gives both answers.
Does it need to use complex numbers for some substitution?
08-27-2016, 06:16 AM
Post: #10
 parisse Senior Member Posts: 1,016 Joined: Dec 2013
RE: CAS: solve solutions
With the current version of Xcas I get -5 and 0 for solve((x+5)*e^x = (x+5)*e^(2*x),x). I don't know why it does not work for older versions of giac.
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