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4.3 Matrices and Determinants

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4.3 Matrices and Determinants

Algebra 2

Mrs. Spitz

Fall 2006

- Evaluate the determinant of a 3 x 3 matrix, and
- Find the area of a triangle given the coordinates of its vertices.

- pp. 170-171 #5-26 all

- Every square matrix has a determinant. The determinant has the same elements as the matrix, but they are enclosed between vertical bars instead of brackets. In Chapter 3, you learned a method for evaluating a 2 x 2 determinant.

- The determinant of
is . To evaluate

The determinant, use the rule for second order determinants.

- A method called expansion by minors can be used to evaluate the determinant of a 3 x 3 matrix. The minor of an element is the determinant formed when the row and column containing that element are deleted. For the determinant

- To use expansion by minors with third-order determinants, each member of one row is multiplied by its minor. The signs of the products alternate, beginning with the second product. The definition below shows an expansion using the elements in the first for of the determinant. However, ANY row can be used.

Expansion of a Third-Order Determinant

Ex. 1: Evaluate the determinant of

Using expansion by minors.

- Another method for evaluating a third order determinant is using diagonals.
- STEP 1: You begin by repeating the first two columns on the right side of the determinant.

- STEP 2: Draw a diagonal from each element in the top row diagonally downward. Find the product of the numbers on each diagonal.

aei

bfg

cdh

- STEP 3: Then draw a diagonal from each element in the bottom row diagonally upward. Find the product of the numbers on each .

gec

hfa

idb

- To find the value of the determinant, add the products in the first set of diagonals, and then subtract the products from the second set of diagonals.
The value is:

aei + bfg + cdh– gec – hfa – idb

First, rewrite the first two columns along side the determinant.

Ex. 2: Evaluate using diagonals.

Next, find the values using the diagonals.

0

-5

24

Ex. 2: Evaluate using diagonals.

4

60

0

Now add the bottom products and subtract the top products.

4 + 60 + 0– 0 – (-5) – 24 = 45. The value of the determinant is 45.

- Determinants can be used to find the area of a triangle when you know the coordinates of the three vertices. The area of a triangle whose vertices have coordinates (a, b), (c, d), (e, f) can be found by using the formula:

and then finding |A|, since the area cannot be negative.

Ex. 3: Find the area of the triangle whose vertices

have coordinates (-4, -1), (3, 2), (4, 6).

How to start: Assign values to a, b, c, d, e, and f and substitute them into the area formula and evaluate.

a = -4, b = -1, c = 3, d = 2, e = 4, f = 6

8

-24

-3

18

-8

-4

Now add the bottom products and subtract the top products.

-8 + (-4) + 18– 8 – (-24) –(-3) = 25. The value of the determinant is 25. Applied to the area formula ½ (25) = 12.5. The area of the triangle is 12.5 square units.

Sometimes one or more of the elements of a determinant may be unknown, but the value of the determinant is known. You can use expansion to find the values of the variable.

Ex. 4: Solve for n if

24n – 84 – 36n + 48n +84n – 18 = -582

120n – 102 = -582

120n = -480

n = -4

The value of n is -4.

Collect like terms

Add 102 to both sides

Divide by 120 both sides.