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Re: use of SetDelayed functions in Plot and Solve


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


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