The calculation of the 3rd order determinant can be done by means of a practical device called *Sarrus's rule.*

Follow how we apply this rule to .

**1st step**: We repeat the first two columns next to the third:

**2nd step**: We find the sum of the product of the elements of *main diagonal* with the two products obtained by multiplying the elements of the parallel to this diagonal (the sum must be preceded by the plus sign):

**3rd step**: We find the sum of the product of the elements of *secondary diagonal* with the two products obtained by multiplying the elements of the parallel to this diagonal (the sum must be preceded by the negative sign):

Like this:

Note: If we develop this 3rd order determinant by applying the Laplace Theorem, we will find the same real number.

## Order determinant n> 3

We have seen that the Sarrus rule is valid for the calculation of the determinant of an order 3 matrix. When the matrix is of order greater than 3, we must employ Laplace's theorem to arrive at order 3 determinants and then apply the Sarrus rule. .

Next: Determinant Properties