1. Determinants
by M. Bourne
Before we see how to use a matrix to solve a set of simultaneous equations, we learn about determinants.
A determinant is a square array of numbers (written within a pair of vertical lines) which represents a certain sum of products.
On the right is an example of a 3 × 3 determinant (it has 3 rows and 3 columns).
The result of multiplying out, then simplifying the elements of a determinant is a single number (a scalar quantity).
Calculating a 2 × 2 Determinant
In general, we find the value of a 2 × 2 determinant with elements a, b, c, d as follows:
We multiply the diagonals (top left × bottom right first), then subtract.
Example:
The final result is a single number.
We will see how to expand a 3 × 3 determinant below.
Let's see how LiveMath does this for us.
Using Determinants to Solve Systems of Equations
We can solve a system of equations using determinants, but it becomes very tedious for large systems. We will only do 2 × 2 and 3 × 3 systems using determinants.
Cramer's Rule.
The solution (x, y) of the system
can be found using determinants:
Example:
Solve the system using Cramer's Rule:
LiveMath can also perform Cramer's Rule for us, as follows. We will see later that LiveMath can solve systems of equations more directly for us.
3 × 3 Determinants
A 3 × 3 determinant
can be evaluated in various ways.
We will use the method called "expansion by minors". But first, we need a definition.
Cofactors
The 2 × 2 determinant
is called the cofactor of a1 for the 3 × 3 determinant:
The cofactor is formed from the elements that are not in the same row as a1 and not in the same column as a1.
Similarly, the determinant
is called the cofactor of a2. It is formed from the elements not in the same row as a2 and not in the same column as a2.
We continue the pattern for the cofactor of a3.
Expansion by Minors
We evaluate our 3 × 3 determinant using expansion by minors. This involves multiplying the elements in the first column of the determinant by the cofactors of those elements. We subtract the middle product and add the final product.
Note that we are working down the first column and multiplying by the cofactor of each element.
Example:
Evaluate
We can use LiveMath to demonstrate expansion by minors. [I have included this so you can check your work on paper. LiveMath evaluates determinants directly, as we see in the next LiveMath example.]
However, now let us see how LiveMath does it directly:
Cramer's Rule to Solve 3 × 3 Systems of Linear Equations
We can solve the system
by using:
where
Example:
Solve, using Cramer's Rule:
In this LiveMath example, I have indicated the final answer only, so you can check your work. Cramer's Rule for larger systems becomes quite tedious.
Determinant Exercises
1. Evaluate by expansion of minors:
2. Solve the system by use of determinants:
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