Constructing Equations for a Circle with 3 Points

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Homework Help Overview

The discussion revolves around constructing the equation of a circle given three specific points. The original poster seeks to determine the coefficients a, b, c, and d in the equation ax² + ay² + bx + cy + d = 0, using the points (-4, 5), (4, -3), and (-2, 7).

Discussion Character

  • Exploratory, Assumption checking

Approaches and Questions Raised

  • The original poster questions whether they need to treat one of the coefficients as a parameter due to having only three equations for four unknowns. They also inquire about the possibility of constructing a fourth equation.
  • Some participants suggest using the standard form of a circle to highlight the dependency of the parameters, indicating that only three parameters are necessary.
  • One participant proposes using Gauss Jordan elimination to express some coefficients in terms of a parameter, questioning if this would suffice.
  • Another participant clarifies that the notion of a fourth parameter may not be necessary, as one can eliminate a parameter by dividing the entire equation by a nonzero value.

Discussion Status

Contextual Notes

Participants are navigating the constraints of having three points and four unknowns, leading to discussions about the independence of the parameters in the context of a circle's equation.

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Homework Statement



Find coefficients a, b, c, d so that the circle with the following 3 points satisfies the equation:

[tex]ax^{2} + ay^{2} + bx + cy + d = 0[/tex]

Points:

(-4, 5)
(4, -3)
(-2, 7)



Homework Equations





The Attempt at a Solution


I'm wondering if since I can only construct 3 equations from the 3 points, if I will have to make one unknown a parameter - probably d.

Is there a way to construct a 4 th equation which I'm missing?

Thanks!
 
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The parameters a,b,c and d are not independent if you are given that it's a circle. Write the equation of a circle in the form (x-a)^2+(y-b)^2=r^2. Now you only have three parameters. And you have three points.
 
Last edited:
What if I used Gauss Jordan elimination to find a,b and c in terms of parameter d, would that sufficiently answer the question?
 
Sure, I suppose. The 'fourth parameter' is really that you can divide your whole equation by anyone of the four parameters that is nonzero and eliminate it. It was never really there to begin with. I.e. x^2+y^2+bx+cy+d=0 is also just as good.
 
Last edited:

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