# Re: Numerical solution from Module

83 views

### Bob Hanlon

May 12, 2014, 12:42:20 AM5/12/14
to

It is a scoping issue which is why the approach that uses explicit
parameters in f works. Alternatively, deconflict the symbols

Clear[a, b, c, f, soln];

f[x_] = (a*x + b)*x + c;

soln[aa_, bb_, cc_] = Module[{x},
x /. Solve[f[x] == 0, x] /.
{a -> aa, b -> bb, c -> cc}];

soln[a, b, c]

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

soln[1, -3, 2]

{1, 2}

Bob Hanlon

On Fri, May 9, 2014 at 2:07 AM, Rob Y. H. Chai <yhc...@ucdavis.edu> wrote:

>
> Hello every one,
>
>
>
> Here is a simple question. Say I define a function
>
>
>
> In:= f[x_] := a*x^2 + b*x + c
>
>
>
> Then I use Module to frame the solution of f[x] ==0
>
>
>
> In:= 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:= soln[1, -3, 2]
>
>
>
> Out= {(-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
>
>
>

### Sseziwa Mukasa

May 12, 2014, 12:45:43 AM5/12/14
to
The arguments of soln are not the a, b, and c in f. A couple of solutions:

soln[aval_, bval_, cval_] := Module[{}, xsoln = Solve[f[x] == 0 , x]; x /.
xsoln/.{a->aval,b->bval,c->cval}]

f[x_,a_,b_,c_]:= a*x^2 + b*x + c

soln[a_,b_,c_]:=x/.Solve[f[x,a,b,c]==0,x]