Sarrus' rule

     Sarrus’s Rule offers a fast and visual way to compute the determinant of a 3 × 3 matrix.

     To do this, we rewrite the first two columns of the matrix next to it. Then:

1. Multiply along the three main diagonals (from top-left to bottom-right) — these are the positive terms.
2. Multiply along the three secondary diagonals (from bottom-left to top-right) — these are the negative terms.
3. Subtract the sum of the negatives from the sum of the positives.

The next diagram illustrates the process:

🔺 Positive diagonals (blue)

🔻 Negative diagonals (red)