Proving Hyperbola for Quadratic Equation (X^T)AX = k

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SUMMARY

The discussion focuses on proving that the graph of the quadratic equation (X^T)AX = k represents a hyperbola when k is non-zero and the determinant of the symmetric matrix A is less than zero. The matrix A is defined as A = \begin{bmatrix} a & b \\ c & d\end{bmatrix}, and the quadratic form expands to ax^2 + (b+c)xy + dy^2. The conditions for this expression to represent a hyperbola are derived from the properties of conic sections, specifically requiring that the discriminant of the quadratic form is positive.

PREREQUISITES
  • Understanding of quadratic forms and conic sections
  • Familiarity with matrix operations, specifically symmetric matrices
  • Knowledge of determinants and their implications in linear algebra
  • Ability to manipulate and analyze polynomial expressions
NEXT STEPS
  • Study the properties of conic sections, focusing on hyperbolas
  • Learn about the conditions for a quadratic form to represent different conic sections
  • Explore the implications of the determinant in relation to matrix eigenvalues
  • Investigate the geometric interpretations of quadratic forms in two dimensions
USEFUL FOR

Students in mathematics, particularly those studying linear algebra and conic sections, as well as educators looking for examples of quadratic forms and their geometric representations.

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Homework Statement



Let A be a 2x2 symmetric matrix and x be a scalar. Prove that the graph of the quadratic equation (X^T)AX = k is hyperbola if k is non zero and det(A) less than zero

(T stands for transpose

Homework Equations



not were to begin
 
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Well, begin by writing it out!
Let X= <x, y> and
[tex]A= \begin{bmatrix} a & b \\ c & d\end{bmatrix}[/tex].

Then
[tex]X^TAX= \begin{bmatrix}x & y\end{bmatrix}\begin{bmatrix}a & b \\ c & d\end{bmatrix}\begin{bmatrix} x \\ y\end{bmatrix}=ax^2+ cxy+ bxy+ dy^2= ax^2+ (b+ c)xy+ dy^2[/tex]

Now, under what conditions on a, b, c, d is [itex]ax^2+ (b+ c)xy+ dy^2[/itex] a hyperbola?
 

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