Numerical solution from Module

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Rob Y. H. Chai

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May 9, 2014, 2:07:46 AM5/9/14
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Hello every one,



Here is a simple question. Say I define a function



In[14]:= f[x_] := a*x^2 + b*x + c



Then I use Module to frame the solution of f[x] ==0



In[15]:= soln[a_, b_, c_] := Module[{}, xsoln = Solve[f[x] == 0 , x]; x /.
xsoln]



When I enter numerical values for parameters a, b, and c in the module, f[x]
never sees these numerical values, and returns a symbolic solution.



In[11]:= soln[1, -3, 2]



Out[11]= {(-b - Sqrt[b^2 - 4 a c])/(2 a), (-b + Sqrt[b^2 - 4 a c])/(2 a)}



But I want the module to return a numerical solution as:

{{x -> 1}, {x -> 2}}



My question is: without bringing f[x] explicitly into the Module function,
and without redefining f as f[a_,b_,c_][x] := a*x^2+b*x+c, how can I get the
module to return a numerical solution?



Thanks - Rob Chai


Kevin

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May 12, 2014, 12:44:02 AM5/12/14
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I would put f inside the module as well. Here is my version:

Clear[soln, a, b, c]
soln[a_, b_, c_] := Module[{xsoln, f, x},
f[x_] = a*x^2 + b*x + c;
xsoln = Solve[f[x] == 0, x]; x /. xsoln
]

Try it out:

soln[1,2,3]

output: {-1-I Sqrt[2],-1+I Sqrt[2]}
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