In matrix theory, the rule of Sarrus is a mnemonic device for computing the determinant of a
matrix named after the French mathematician Pierre Frédéric Sarrus.
Then add the products of the diagonals going from top to bottom (solid) and subtract the products of the diagonals going from bottom to top (dashed).
This yields[1][2] A similar scheme based on diagonals works for
[1] Another way of thinking of Sarrus' rule is to imagine that the matrix is wrapped around a cylinder, such that the right and left edges are joined.