CAS diff() giving incorrect results - Printable Version +- HP Forums (https://www.hpmuseum.org/forum) +-- Forum: HP Calculators (and very old HP Computers) (/forum-3.html) +--- Forum: HP Prime (/forum-5.html) +--- Thread: CAS diff() giving incorrect results (/thread-17507.html) CAS diff() giving incorrect results - Wes Loewer - 09-22-2021 07:10 PM A couple of years ago I posted about a problem with the diff() command (https://www.hpmuseum.org/forum/thread-13532.html). The problem still exists in the most recent firmware. I guess I had forgotten about the issue because I ran into the problem again yesterday, only it's worse than I original thought. The problem arises when you try to evaluate a derivative at a value, which is something that frequently comes up in calculus and physics. For instance, evaluating the 2nd derivative of x^3 at x=4. diff(x^3,x,2)|x = 4 which should evaluate to 6*x|x=4 which is 24. However, the CAS substitutes 4 in for x BEFORE differentiating, so the expression is interpreted as diff(4^3,4,2) = diff(64,4,2) which is meaningless. Even worse than no answer is giving the wrong answer. If the value of x contains pi, then you get incorrect results. diff(x^3+π^4,x)|x = π should result in 3*x^2|x=π which is 3*π^2. Instead, the line is interpreted as diff(π^3+π^4,π) and now π is treated as a variable. So the result is 3*π^2+4*π^3. This issue came to light yesterday when I was checking my test answer key and entered this (using primes which is equivalent to diff). (x^2*cos(x))''|x = π evaluated to -2 instead of π^2-2. I assumed that I had made some silly mistake in my work by hand. It took several repeated attempts before I concluded that I was correct and the calculator was wrong. I realize I could use the d/dx template which works correctly, but there is no template for 2nd or higher order derivatives and (∂(∂(x^2*cos(x),x),x)|(x = π)) looks really awkward (especially in Textbook mode) and using the ' (primes) is so very convenient. It's really an order of operations issue in which |x=a should be applied AFTER the diff() command has completed, not BEFORE. And it seems only natural that the HP "Prime" should be able to correctly work with "primes". RE: CAS diff() giving incorrect results - parisse - 09-22-2021 07:26 PM I have already tried to make | work as you expect with diff, but that was not successfull, it's too complicated. Please use subst Code: ``` subst(diff(x^3,x,2),x = 4)``` or just parentheses like Code: ``` diff(x^3,x,2)(x = 4)``` RE: CAS diff() giving incorrect results - Wes Loewer - 09-23-2021 06:20 PM (09-22-2021 07:26 PM)parisse Wrote:  I have already tried to make | work as you expect with diff, but that was not successfull, it's too complicated. Please use subst Okay, thanks for that info. That's good to know. Quote:or just parentheses like Code: ``` diff(x^3,x,2)(x = 4)``` That's an interesting notation. It appears to work for other functions as well. Is there any way to use it with primes? When I try (x^3)''(x = 4) it gives a warning and returns (6*x) = 24, with the expression on the left and the result on the right. Is this the intended behavior? RE: CAS diff() giving incorrect results - toml_12953 - 09-23-2021 07:26 PM (09-23-2021 06:20 PM)Wes Loewer Wrote:   (09-22-2021 07:26 PM)parisse Wrote:  I have already tried to make | work as you expect with diff, but that was not successfull, it's too complicated. Please use subst Okay, thanks for that info. That's good to know. Quote:or just parentheses like Code: ``` diff(x^3,x,2)(x = 4)``` That's an interesting notation. It appears to work for other functions as well. Is there any way to use it with primes? When I try (x^3)''(x = 4) it gives a warning and returns (6*x) = 24, with the expression on the left and the result on the right. Is this the intended behavior? For that expression, the pipe does work (x^3) | (x = 4) 64 RE: CAS diff() giving incorrect results - Albert Chan - 09-24-2021 10:48 AM (09-23-2021 06:20 PM)Wes Loewer Wrote:  When I try (x^3)''(x = 4) it gives a warning and returns (6*x) = 24, When I try same expression in the emulator, above return only 24, without warning. I beiieve (x^3)''(x = 4) equivalent to subst((x^3)'', x = 4), only less typing. Quote:That's an interesting notation. It appears to work for other functions as well. If the expression is not a function, the notation may get confused with implied multiply. XCAS> (1+x+x^2) (x=0.1) Warning : using implicit multiplication for (1+x+x^2)(x=0.1) x*(x^2+x+1)=(0.1*(x^2+x+1)) CAS> (1+x+x^2) (x=0.1) ﻿ ﻿ ﻿ ﻿ ﻿ ﻿ ﻿ ﻿ → "Syntax Error" We can get around the problem (both CAS and XCAS) with identity function. CAS> id(1+x+x^2) (x=0.1) ﻿ ﻿ ﻿ ﻿ ﻿ ﻿ → 1.11 RE: CAS diff() giving incorrect results - Wes Loewer - 09-24-2021 04:28 PM (09-24-2021 10:48 AM)Albert Chan Wrote:   (09-23-2021 06:20 PM)Wes Loewer Wrote:  When I try (x^3)''(x = 4) it gives a warning and returns (6*x) = 24, When I try same expression in the emulator, above return only 24, without warning. I beiieve (x^3)''(x = 4) equivalent to subst((x^3)'', x = 4), only less typing. That's interesting. That's exactly what I was hoping to accomplish. What version are you using? I tried this on the following versions: 2020-01-16 (HW-A, VC) no warning, returns 24 2020-09-08 (HW-A), 2021-05-05 (HW-D), 2021-06-09 (VC) has warning, returns 6*x=24 RE: CAS diff() giving incorrect results - Albert Chan - 09-24-2021 05:23 PM Hi, Wes Loewer It was an old version: Build: 2.1.14181 (2018 10 16) RE: CAS diff() giving incorrect results - Wes Loewer - 09-24-2021 07:59 PM (09-24-2021 05:23 PM)Albert Chan Wrote:  It was an old version: Build: 2.1.14181 (2018 10 16) Okay, I guess something changed since then. RE: CAS diff() giving incorrect results - parisse - 09-25-2021 04:04 PM f(x=...) will work if f is an expression. It will not work with e.g. (x^3)'' because the parser will not detect that it's an expression (and it will do like if it was implicit multiplication because x appears in both). If you work with derivatives, it's a good idea to save them in variables. First f:=x^3'', then f(x=4) RE: CAS diff() giving incorrect results - Albert Chan - 09-25-2021 05:03 PM (09-23-2021 06:20 PM)Wes Loewer Wrote:  When I try (x^3)''(x = 4) it gives a warning and returns (6*x) = 24 (09-25-2021 04:04 PM)parisse Wrote:  f(x=...) will work if f is an expression. It will not work with e.g. (x^3)'' because the parser will not detect that it's an expression (and it will do like if it was implicit multiplication because x appears in both). Why does older version detected the expression, but not newer version ? Does this mean newer versions of HP Prime CAS support implied multiplication ? It still does not explain Wes Loewer weird result. Assuming parser get confused, assumed implicit multiplication, we should have: CAS> (6*x) * (x = 4) ﻿ ﻿ ﻿ ﻿ ﻿ ﻿ → 6*x^2 = 24*x RE: CAS diff() giving incorrect results - Albert Chan - 09-26-2021 12:04 PM I thought that expression|(x=a) notation can be simulated with eval(subst(quote(expression), x=a)) But, not quite. subst version is better ! (I skipped eval, because f(x) not defined) CAS> 'integrate(f(x),x)' (x = t*t integrate(f(t*t)*(t+t),t) CAS> 'integrate(f(x),x, 1, 2)' (x = t*t integrate(f(t*t)*(t+t),t, 1, √2) CAS> integrate(f(x),x) |x = t*t x*f(t^2) CAS> integrate(f(x),x, 1, 2) |x = t*t integrate(f(x),x, 1, 2) --- I may have found a quoting bug. If quoted expression is saved to a variable, quote is gone. CAS> assume( t >= 0) CAS> m := 'integrate(1/(√x*(x+1)),x)' (x = t*t) integrate(1/(√(t*t)*(t*t+1))*(t+t),t) CAS> m // should have matched Ans 2*atan(t) Update: another bug, if we use notation x = a .. b CAS> 'integrate(f(x),x = 1 .. 2)' (x = t*t integrate(f(t*t),(t*t) = (1 .. 2))