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Defining a function f[x_] := from InputField (How??)

Posted 9 years ago
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I just started using Mathematica in February, learning & enjoying (when I can). Recently i've just began exploring the manipulate function, while the manipulate part itself works fine, I have to manually change the f[x_] for each equation I want to explore. In order to export to CDF or just simplify the looks I wanted to use an InputField or similar method to express an equation but I can't seem to figure out how to go from InputField to an equation the manipulation expression will work with. The working code I have is:
f[x_] := {{XXXX}}
g[a_] := Evaluate[D[f[a], a]]
Manipulate[Plot[{f[x], g[x], g[a] *(x-a) + f[a]}, {x, c,-1*c},
PlotRange -> {b, -1*b},
PlotLedgends -> {f[x], g[x], "f'(x) m value" -> g[a]}], {{a, -5}, {-10,10},
{{b,5}, 0.1,10}, {{c, 5}, .1,10}]
//Replace {{XXXX}} the equation

All it does is plot a function, its derivative and the tangent line that tracks the original function. I was trying to define the function via InputField and despite my best efforts I can't crack it.
POSTED BY: Zach Wolfe
    {"Equation", Row[{u, " = 0"}]},
    {"Solutions", Solve[u == 0, x]},
    {"Roots", Roots[u == 0, x]},
    {"Discriminant", Discriminant[u, x]},
    {Plot[u, {x, -5, 5}]}
  Alignment -> Left],
{{u, x^2 + 2 x + 1, "Equation"}},
BaseStyle -> Directive[
   FontSize -> 16,
   FontFamily -> "Optima"]

Is this related to your question? 

I don't think you can make a CDF out of this, though.
POSTED BY: C ormullion
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