How does one make a function in Mathematica to later call and solve it?

Question

Such as if I wanted to make a quadratic function, save it, and then later plug in any values of a,b,c in order to solve for x.

This was what I was thinking it would be:

fQ[a_,b_,c_]:=a x^2+b x+c ==0

Then I want to be able to call it and solve it with arbitrary values

Solve[fQ[1,2,3],x]

I then get this:

Set::write: Tag Plus in 3+2 x+x^2 is Protected.

Solve::eqf: 0 is not a well-formed equation.

Thank you

@Kuba Meant to have originally wrote that in the question. Has been corrected. — Jwizz, Mar 19 '18 at 20:46
Duplicate: mathematica.stackexchange.com/questions/169311/… — David G. Stork, Mar 19 '18 at 22:23

David G. Stork · Accepted Answer · 2018-03-19 20:46:53Z

1

mySol[a_, b_, c_] := Solve[a x^2 + b x + c == 0, x]

mySol[1, 2, 3]

$\left\{\left\{x\to -1-i \sqrt{2}\right\},\left\{x\to -1+i \sqrt{2}\right\}\right\}$

Your problem was that you tried to include x, which is irrelevant.

answered Mar 19 '18 at 20:46

David G. Stork

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$\begingroup$ Thank you so much! $\endgroup$ – Jwizz Mar 19 '18 at 20:48
$\begingroup$ What if with the x^2 and the x I wanted to multiply it by the identity matix 5? I thought it might be: aFunc[a_, b_, c_] := Solve[a x^2*(Times[IdentityMatrix[5]]) + b x*(Times[IdentityMatrix[5]]) + c == 0, x] $\endgroup$ – Jwizz Mar 19 '18 at 20:51
$\begingroup$ However, the output is giving me Out[]={} when I type in aFunc[1,2,3] $\endgroup$ – Jwizz Mar 19 '18 at 20:52
$\begingroup$ If you try to mix matrices (IdentityMatrix[]) and scalars (c) you don't have a valid equation. Moreover, this extension is not part of the question as stated and it is very bad etiquette indeed to ask a question, get a full answer, not accept or up-vote it, and then say... oh... but what I really wanted to ask is... $\endgroup$ – David G. Stork Mar 19 '18 at 20:54
$\begingroup$ Apologies for doing that. It was a two part question. I should have merged into one. $\endgroup$ – Jwizz Mar 19 '18 at 20:55

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