subst() vs |x=a with diff() - 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: subst() vs |x=a with diff() (/thread-13532.html) subst() vs |x=a with diff() - Wes Loewer - 08-29-2019 05:05 PM I ran into an issue that I mostly sorted out and thought others might benefit from the experience. I was attempting to find a 2nd derivative at a point so I tried diff(t*e^t,t,2)|t=3 but the t=3 is substituted first, so I got diff(3*e^3,3,2) I was able to use the following work around ∂(∂(t*e^t,t),t)|t=3 but it's not exactly elegant (looks even worse in Textbook display), and it doesn't scale easily to higher order derivatives. The problem is the |t=3 substitution occurs before the diff() command is run. I found that the subst() command makes the substitution after its expression is evaluated, so this works as intended. subst(diff(t*e^t,t,2),t,3) Since something like ∂(sin(x),x)|x=2 does work, I guess that ∂() has a higher evaluation order than |x=2. Seems like diff() should be given this same behavior.