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subst() vs |x=a with diff()
08-29-2019, 05:05 PM
Post: #1
subst() vs |x=a with diff()
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


but the t=3 is substituted first, so I got


I was able to use the following work around


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.


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.
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