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I found this 4x4 matrix on wikipedia. It's determinant is used to tell if a vector D in R2 lies inside, outside or on the circumcircle of a triangle ABC.

4x4 matrix determinant

What I fail to understand is:

Why does the determinant of the 4x4 matrix equal the determinant of the simpler 3x3 matrix and why does it seem to make no difference if you say (A^2 - D^2) or (A - D)^2 in this case?

Thanks for your answers, I'm just beginning to learn linear algebra.

Have a nice day!

PS

this is the link to the wikipedia article https://en.wikipedia.org/wiki/Delaunay_triangulation

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1 Answer 1

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  1. Subtracting one row from another does not change the determinant. Apply this rule three times to see that subtracting the last row from the others leaves a matrix whose last column has three $0$ and one $1$. Then apply Laplace expansion on the last column.

  2. Subtract $2D_x$ times the first column from the third column. Subtract $2D_y$ times the second column from the third column.

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  • $\begingroup$ which rule does this follow? I thought you can only substract one row from another. $\endgroup$
    – bakkaa
    Sep 26, 2017 at 11:25
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    $\begingroup$ @Peter It’s the second point of his reply, $(A - D)^2 = A^2 - D^2 - 2D(A - D)$. $\endgroup$
    – k.stm
    Sep 26, 2017 at 11:49
  • $\begingroup$ @k.stm I see, thank you. $\endgroup$
    – Peter
    Sep 26, 2017 at 11:53

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