Re: use of SetDelayed functions in Plot and Solve

*To*: mathgroup at smc.vnet.net*Subject*: [mg44616] Re: use of SetDelayed functions in Plot and Solve*From*: Jens-Peer Kuska <kuska at informatik.uni-leipzig.de>*Date*: Tue, 18 Nov 2003 06:41:45 -0500 (EST)*Organization*: Universitaet Leipzig*References*: <bp4jve$b8p$1@smc.vnet.net>*Reply-to*: kuska at informatik.uni-leipzig.de*Sender*: owner-wri-mathgroup at wolfram.com

Hi, the result of a numeric operation like NIntegrate[] is a number, you can't build the derivative of it. The result of a symbolic operation like Integrate[] is an expression and you can build the derivative. Regards Jens Mukhtar Bekkali wrote: > > Why can't I plot the function and find the solution to the equation below > using SetDelayed functions? Below is just an example, the real functions > are symbolically non-integrable. > > s[t_] :=NIntegrate[t^2 - z*Log[t] , {z, 0, k[t]}] > i[t_] := D[s[t],t] /. {k[t] ->2t,k'[t]->Sin[t]} > Plot[i[t], {t, 1, 10}] > Solve[i[t]==300, t]] > > All I get is an empty graph because I have "not machine real size numbers" > and my equation "appears to be solved essentially in non-algebraic way". > If I rewrite the code using simple Set I get my graph and solutions. > > s=Integrate[t^2 - z*Log[t] , {z, 0, k[t]}] > i=D[s,t] /. {k[t]->2t,k'[t]->Sin[t]} > Plot[i, {t, 1, 10}] > Solve[i==300, t]] > > Very confused. Thanks, Mukhtar Bekkali