Example

Example: Testing for Collinear Points Using Determinants

To determine whether three points are collinear, substitute their coordinates into a 3imes33 imes 3 determinant with a column of ones. For example, to check the points (5,5)(5, -5), (4,3)(4, -3), and (3,1)(3, -1), evaluate: 551431311\begin{vmatrix} 5 & -5 & 1 \\ 4 & -3 & 1 \\ 3 & -1 & 1 \end{vmatrix}. Expanding by minors along the third column yields 1(4(1)3(3))1(5(1)3(5))+1(5(3)4(5))=1(4+9)1(5+15)+1(15+20)=510+5=01(4(-1) - 3(-3)) - 1(5(-1) - 3(-5)) + 1(5(-3) - 4(-5)) = 1(-4 + 9) - 1(-5 + 15) + 1(-15 + 20) = 5 - 10 + 5 = 0. Since the determinant equals 00, the points are collinear.

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Updated 2026-05-26

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