HP PRIME: derivative at a point Alasdair McAndrew Unregistered Posts: 12 Threads: 5 Joined: Nov 2013 11-18-2013, 10:37 PM I was experimenting with some interpolation techniques; in the CAS view of the Functions app I've entered: ```{-3,1,2,5} -> xs {-61,-5,-1,83} -> ys product(x-xs) -> q(x) ``` So far so good. Now what I want to do is evaluate the derivative of q(x) at all the elements of xs. Here's where I get stuck: "diff" only computes symbolic derivatives, and SLOPE doesn't seem to work: ```SLOPE(q(x),7) SLOPE(q(X),7) ``` return 0 and "Error: Undefined Result" respectively. q(x) returns (3+x)*(-1+x)*(-2+x)*(-5+x) as it should q(X) returns 0 - it seems to think that the value of X is 0 - and this persists even after "purge(X)". In Home view, q(x) returns Error: Syntax Error q(X) returns -30 So no joy there. I have discovered that, for example ```eval(diff(q(x)),x=3) ``` will, for example, compute the derivative at a point. But it won't let me pick out a list element: ```eval(diff(q(x)),x=xs(1)) ``` produces a long expression full of "xs(1)": clearly for some reason this is not treated as a numeric value. The best I can do is ```seq(eval(diff(q(x)),x=k),k=xs) -> ds ``` which works, but is confusing. (MAKELIST doesn't seem to work here: in fact MAKELIST hardly even seems to work). Are there any "natural" ways to perform these sort of computations? [Note, on both the TI-nspire and the Casio ClassPad I would just use "diff(q(x)|(x=xs)".] Edited: 18 Nov 2013, 10:39 p.m. Tim Wessman Unregistered Posts: 1,278 Threads: 44 Joined: Jul 2007 11-18-2013, 10:58 PM dq(x)/dx=xs or (dq(x)/dx)|x=xs both work I beleive. X exists because it is a non-removable home real variable. Also, there is not any such thing as a HOME or CAS view. There is 1 CAS view, and 1 home view. The application is "active" while you are in those views, but it is not attached to it in any way. parisse Unregistered Posts: 38 Threads: 0 Joined: Aug 2013 11-19-2013, 06:52 AM map(xs,q') or apply(q',xs) Edited: 19 Nov 2013, 6:58 a.m. « Next Oldest | Next Newest »

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