How does the discriminant work (quadratics)

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Discussion Overview

The discussion revolves around the use of the discriminant in quadratic equations, particularly in the context of finding unknown coefficients and determining the conditions under which a quadratic has specific properties, such as the number of real roots. The scope includes theoretical exploration and application of the discriminant in solving quadratic equations.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant recalls using the discriminant to find unknown coefficients in quadratic equations and questions why this method works.
  • Another participant explains that the discriminant indicates the nature of the roots of a quadratic (real vs. complex) and relates this to applications in calculus.
  • A participant suggests that the original poster (OP) may be referring to problems that involve determining the range of a parameter that affects the number of real roots, providing an example involving a parameter "k".
  • Further clarification is provided that finding the range of parameters can be viewed as adding constraints to reach a solution, likening it to tracing back through conditions to find a root problem.
  • The OP expresses a desire for a deeper mathematical understanding of the topic, indicating a level of expectation for more complex insights.

Areas of Agreement / Disagreement

Participants generally agree on the role of the discriminant in determining the nature of roots in quadratics, but there is no consensus on the deeper mathematical implications or the specific types of problems being discussed.

Contextual Notes

Some assumptions about the nature of the quadratics and the specific problems involving parameters remain unresolved, as do the implications of the discriminant in more complex scenarios.

autodidude
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I remember back when I did a lot of quadratic equations, occasionally I had to find an unknown coefficient and if I remember correctly, generally I had to use the discriminant which gave me another quadratic which I could then use to solve for the coefficient.

My question is why does this work? Why can the discriminant be used in this way?
 
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Hey autodidude.

The discriminant in a quadratic well tell you whether the quadratic has two real roots (discriminant > 0), one real root (discriminant = 0) or no real roots (two complex roots with discriminant < 0).

The discriminant is a nice way to show the above and in certain applications, the nature of the above affects results that build on the result of solving these including things in calculus like solving linear ordinary differential equations.

Basically you can show by completing the square that the solution can be found in terms of the discriminant, but the discriminant is usually just an explicit expression that can be evaluated with a set of given constants. This is the core of why the quadratic equation works.

I get a feeling though you are not talking about an ordinary quadratic in the "special occasions". Maybe you could outline the sort of problem you are referring to if the above is not the case?
 
I think the OP is referring to questions involving determining the range of a parameter in one more more of the coefficients that result in a particular outcome for the number of real roots.

Something for example like: Find the range of parameter "k" such that the quadratic, [itex]x^2 + (2-k)x + 4[/itex], is positive definite.

Is that the type of problem you were thinking of autodidude?
 
Last edited:
Ohh ok.

Well in that case it's just adding an extra constraint which means you find that constraint to find your original problem.

Think of it like a tree where the leaves of the tree are all the conditions that you have and you work all the way back to the root to get a final solution.

It's more or less just a means to an end.
 
uart said:
I think the OP is referring to questions involving determining the range of a parameter in one more more of the coefficients that result in a particular outcome for the number of real roots.

Something for example like: Find the range of parameter "k" such that the quadratic, [itex]x^2 + (2-k)x + 4[/itex], is positive definite.

Is that the type of problem you were thinking of autodidude?

Yep, thanks for posting that!

@chiro: Thanks...that makes sense. I think I was expecting some deeper mathematical result lol
 

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