Past exam of the year 1 of the computer science course of the University of Oxford / 2014 / Digital systems, linear algebra, introduction to formal proof / 6 / d / Solution

A triangular matrix has determinant equal to the product of its diagonal entries. Since $L$ is a unit lower triangular matrix, $\det L=1$. ThusEquivalently, the elimination used only row additions, which leave the determinant unchanged; there were no row swaps or row scalings to account for.