How do I find the coordinates of symmetric matrices after finding the eigenvalues, eigenvectors and eigenbases? So, let's say that I have a quadratic function: 6x_1^2+4x_1x_2+3x_2^2.

Siemensueqw

Siemensueqw

Answered question

2022-11-07

How do I find the coordinates of symmetric matrices after finding the eigenvalues, eigenvectors and eigenbases?
I've been playing around with Symmetric matrices and orthogonal bases of said Symmetric matrices, but I cannot figure out how to find the coordinates.
So, let's say that I have a quadratic function:
6 x 1 2 + 4 x 1 x 2 + 3 x 2 2
Well now I know that this forms a matrix of       A =   ( 6 2 2 3 )
So now I have to find the eigenvalues of this matrix which are λ 1 = 7 , λ 2 = 2.
and now I have two Eigenbases of
E 7 = s p a n ( ( 2 1 ) ) , E 2 = s p a n ( (       1 2 ) )
and the orthogonal bases of these two are just the lengths multiplied by the matrices, so
w 1 = 1 5 ( 2 1 ) w 2 = 1 5 (       1 2 )
But the problem I'm having is that I need an equation q ( w 1   c 1 + w 2   c 2 ) = c 1 2 + c 2 2
But how do I find these coordinates? I read my book and it wasn't clear on what I'm supposed to be doing to obtain them.

Answer & Explanation

lesinetzgl5

lesinetzgl5

Beginner2022-11-08Added 18 answers

Step 1
The quadratic form is written
v T M v
and after diagonalization
v T P Λ P T v
or ( P T v ) T Λ ( P T v ) .
Step 2
So with
w := P T v
the form reduces to
w T Λ w .
Finally
λ 1 w 1 2 + λ 2 w 2 2 = ( λ 1 w 1 ) 2 + ( λ 2 w 2 ) 2 = c 1 2 + c 2 2 .

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