SOLVE
05-21-2022, 11:46 AM
Post: #1
 ks2000 Junior Member Posts: 2 Joined: May 2022
SOLVE
I am using solve function in CAS.

This is the function that i have inputted:

solve((18*log10((12*(2*x+3*x^2)/(2+2*√(10)*x)/0.4))*√(((2*x+3*x^2)/(2+2*√(10)*x))*0.03)*(2*x+3*x^2)-15) = 0,x)

The answer I am given is: [[]]

Similarly when I input:

solve(((2*x+x^2)^(5/3)*√(3/100)/(0.025*(2+2*√(2)*x)^(2/3))) = 15,x)

The answer I am given is: {}

A normal CASIO fx-991ZA PLUS is able to solve for x in this equation, but I am struggling to understand why the HP Prime can't?

Assistance willl be gretaly appreciated!
05-22-2022, 06:32 AM
Post: #2
 rawi Member Posts: 178 Joined: Nov 2019
RE: SOLVE
Hi,

your problems are strictly numeric, so there is no reason to use CAS.
I used the solve app to solve your 2nd problem and it went without any problems and delivered the result 0.9952 which seems to be correct.
05-22-2022, 07:31 AM
Post: #3
 Joe Horn Senior Member Posts: 2,011 Joined: Dec 2013
RE: SOLVE
Suggestion: CAS is optimized for exact and symbolic math, not for approximate math, which is what Home is optimized for. Introducing decimal points anywhere in your command line tells CAS to assume that the inputs are approximate, not exact. Therefore, replace the floating-point values in your expression with exact values, such as replacing "/0.4" with "*5/2", and "*0.03" with "*3/100". When I do this for your expression, CAS yields these two solutions: −0.318312076607 and 0.995115519271

<0|ɸ|0>
-Joe-
05-22-2022, 10:23 AM
Post: #4
 jonmoore Senior Member Posts: 311 Joined: Apr 2020
RE: SOLVE
Further to the other comments, it's important to remember that the Casio model that you're comparing to the HP Prime only works numerically. Much as it has a Solve function, it can only solve numerically, so you're comparing apples with oranges.

One of the nice things about the Prime is that it effectively features two completely separate calculation modes. One for numerical operations and one for symbolic computer algebra operations. However, you have to be careful when calling CAS functions from Home (the numeric Prime personality) as you're not always going to get the answers you expect.

The great thing about the family of Casio calculators you reference (the current model being the fx-991ex), is that they default to exact values. But it's easy to confuse an exact value from the Casio solver as being symbolically derived because it will often match the result from a symbolic solver. But the underlying methodology used by the calculator is very different and has a greater tendency to resolve to approximate answers.
05-22-2022, 11:35 AM
Post: #5
 Steve Simpkin Senior Member Posts: 1,234 Joined: Dec 2013
RE: SOLVE
(05-22-2022 06:32 AM)rawi Wrote:  Hi,

your problems are strictly numeric, so there is no reason to use CAS.
I used the solve app to solve your 2nd problem and it went without any problems and delivered the result 0.9952 which seems to be correct.

For reference, here is a screenshot of the example rawi mentioned using the Solve App solution.

Attached File(s) Thumbnail(s)

05-24-2022, 05:02 AM
Post: #6
 parisse Senior Member Posts: 1,327 Joined: Dec 2013
RE: SOLVE
I get {0.995...}
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