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