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== Example (2 x 2 Matrix) == | == Example (2 x 2 Matrix) == | ||

− | <math>\left[\begin{array}{cccc}4&2\\-3&1\end{array}\right]</math> | + | A = <math>\left[\begin{array}{cccc}4&2\\-3&1\end{array}\right]</math> |

− | + | ||

− | + | ||

det(A) = (4)*(1) - (2)*(-3) | det(A) = (4)*(1) - (2)*(-3) |

## Revision as of 08:15, 9 December 2012

## Contents

## THE DETERMINANT OF A MATRIX

## Definitions

Square Matrix: A matrix whose number of columns is equivalent to its number of rows. (n x n)

**2 x 2 Matrices**

**3 x 3 Matrices**

**4 x 4 Matrices**

## Example (2 x 2 Matrix)

A = $ \left[\begin{array}{cccc}4&2\\-3&1\end{array}\right] $

det(A) = (4)*(1) - (2)*(-3)

= 4-(-6) = 10

## Example (3 x 3 Matrix)

A = [2, 5, 1 ; 3, 2, 2 ; 1, 5, 2]

det(A) = 2*[(2*2) - (2*5)] - 5*[(3*2) - (2*1)] + 1*[(3*5) - (2*1)]

= 2*(4 - 10) - 5*(6 - 2) + 1*(15 - 2) = 2*(-6) - 5*(4) + 1*(13) = -12 - 20 + 13 = -19

## Example (4 x 4 Matrix)

A = [2, 3, 7, 2 ; 1, 6, 2, 3 ; 1, 1, 4, 3 ; 4, 6, 5, 1]

det(A) = 2*{6*[(4*1) - (3*5)] - 2*[(1*1) - (3*6)] + 3*[(1*5) - (4*6)]} - 3*{1*[(4*1) - (3*5)] - 2*[(1*1) - (3*4)] + 3*[(1*5) - (4*4)]} + 7*{1*[(1*1) - (3*6)] - 6*[(1*1) - (3*4)] + 3*[(1*6) - (1*4)]} - 2*{1*[(1*5) - (4*6)] - 6*[(1*5) - (4*4)] + 2*[(1*6) - (1*4)]}

= 2*[6*(4-15) - 2*(1-18) + 3*(5-24)] - 3*[1*(4-15) - 2*(1-12) + 3*(5-16)] + 7*[1*(1-18) - 6*(1-12) + 3*(6-4)] - 2*[1*(5-24) - 6*(5-16) + 2*(6-4)] = 2*[(6*(-11)) - (2*(-17)) + (3*(-19))] - 3*[(1*(-11)) - (2*(-11)) + (3*(-11))] + 7*[(1*(-17)) - (6*(-11)) + (3*2)] - 2*[(1*(-19)) - (6*(-11)) + (2*2)] = 2*(-66 - (-34) + (-57)) - 3*(-11 - (-22) + (-33)) + 7*((-17) - (-66) + 6) - 2*((-19) - (-66) + 4) = 2*(-89) - 3*(-22) + 7*(55) - 2*(51) = -178 - (-66) + 385 - 102 = 171

## Important Properties of the Determinant

- The determinant of the transpose of a matrix will be the same as the determinant of the original matrix.

- If two rows in the matrix are interchanged, the determinant will be the negative of the determinant of the original matrix.

- If two rows or columns in a matrix are the same, the determinant will be zero.

- If there is a row of zeros in the matrix, the determinant will be zero.

- If a row or column in a matrix was multiplied by a real number, then the determinant will be that real number multiplied by the determinant of the original matrix.

- If the matrix is either an upper or a lower triangular matrix, the determinant will be the product of the elements on the main diagonal.

- If two matrices are n x n, then the determinant of one multiplied by the determinant of the other is the determinant of the full matrix of the two next to each other.

## Nonsingular and Similar Matrices

Definition: A square matrix is called **nonsingular** if its determinant is not equal to zero.

- For square matrices, then the linear combination of the matrix has a nontrivial solution if and only if the matrix is NOT nonsingular.

- For nonsingular matrices, the determinant of the inverse of that matrix is equal to 1 over the determinant of the matrix.

- If two matrices are similar matrices, then the determinants of the two are equal.