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13.3 Using Cramer’s Rule to Solve Systems
Now that we can solve 2x2 and 3x3 systems of equations, we want to learn another technique for
solving these systems. This new technique will require us to get familiar with several new
concepts. Let’s start with the following definition.
Definitions: Matrix
A matrix is a rectangular array of numbers written inside of brackets.
Each number in a matrix is called an entry , each horizontal set of numbers is called a row and
each vertical set of numbers is called a column.
Matrices come in a wide variety of sizes. When writing the size of a matrix, we always list the
rows first. So a 2x3 matrix would have 2 rows and 3 columns, for example.
Let’s get a handle on these ideas with the following example.
Example1:
List the entries of the following matrix. What is the 3
rd entry of the 2
nd row and the 1
st entry of the
3
rd column?
Solution:
Since this matrix has 3 rows and 5 columns, it is a 3x5 matrix.
So, with this in mind, the 3
rd entry of the 2
nd row would be a 2, and the 1
st entry of the 3
rd column
would be -2.
With the basic idea of a matrix now down, we need to talk about a couple of different kinds of
matrices. Namely, the coefficient matrix and the augmented matrix.
Definition: Coefficient Matrix
A matrix which is composed of all of the coefficients of a system of equations but does not include
the constant terms.
Definition: Augmented Matrix
A matrix which is composed of all coefficients and constants of a system of equations.
These are fairly easy to find as we see in the next example.
Example 2:
Write the coefficient matrix and the augmented matrix of the following systems.
a. 9 8 0
x y
x y b.
3 2 4
x y
y z
x y z
Solution:
a. The coefficient matrix is the matrix that is generated by coefficients, but not the constant terms
of the given system. The augmented matrix is generated by the coefficients and the constants of
the system. So this means we must have
x y
x y
Coefficient Matrix Augmented Matrix
b. Like in part a. above, we generate the coefficient matrix by only the coefficients and the
augmented matrix by using the entire system. This gives us
x y
y z
x y z
Coefficient Matrix Augmented Matrix
There are a number of operations and other things we can do with a matrix, but for the sake of
simplicity, let’s take a look at only the operation that we need for solving systems. That is, the
operation of the determinant.
Definition: Determinant
A number associated with a square matrix. This number has numerous uses in a variety of
applications.
More important than the definition of a determinant is how we find a determinant. It varies slightly
depending on the size of your matrix. We will only look at evaluating a 2x2 and a 3x
determinant since that is the size of the systems we will be solving.
Evaluating a Determinant
Determinant of a 2x2 matrix:
The determinant of the matrix
2 2
1 1
a b
a b A is given by
2 2
1 1 det ab ab a b
a b A A
Determinant of a 3x3 matrix:
The determinant of the matrix
3 3 3
2 2 2
1 1 1
a b c
a b c
a b c
A is given by
3 3
2 2 1 3 3
2 2 1 3 3
2 2 1
3 3 3
2 2 2
1 1 1
det a b
a b c a c
a c b b c
b c a
a b c
a b c
a b c
A A
Although there are a number of ways to calculate a 3x3 determinant, for the sake of simplicity, we
are only giving one general formula. The reader may reference a precalculus, or linear algebra
book for further development.
d. Again, we will follow the formula
The determinant is a very powerful tool in matrices and can to numerous things. However, we
are only interested in using the determinant to solve systems of equations.
To do this we use something called Cramer’s Rule. This rule is named after 16
th century Swiss
mathematician Gabriel Cramer.
Cramer’s Rule for Solving 2x2 Systems
Consider the system
2 2 2
1 1 1
a x by c
ax by c
Let the three determinants D , Dx and Dy be defined as
2 2
1 1
a b
a b D 2 2
1 1
c b
c b Dx 2 2
1 1
a c
a c Dy
Then, if D 0 , the system has a unique solution of
D
D
x
x D
D
y
y
Although Cramer’s rule seems complicated, it’s merely a matter of computing the coefficient
matrix determinant and then computing that same determinant where each column is replaced by
the constants in the system. Then, generating the fractions to get the solution.
Let’s look at it with the next example.
Example 4:
Solve the system by using Cramer’s Rule.
a. 3 7 5
x y
x y b. 1
x y
x y
Solution:
a. The first this we need to do is determine all of the determinants D , Dx and Dy. We use the
formulas given above to do so as follows.
2 2
1 1
a b
a b D 2 2
1 1
c b
c b Dx 2 2
1 1
a c
a c Dy
D
Dx 3 5
Dy
This gives
D
Dx
Dy
So then we just need to evaluate D
D
x
x and D
D
y
y to get our solution. These give
D
D
x
x and 2
D
D
y
y
So the solution is
.
b. As in part a. above, we need to determine the determinants in order to solve the system.
This gives
D
Dx
Dy
Calculating x and y we get 3 1
D
D
x
x and 2 1
D
D
y
y
So the solution is (-3, 2).
Now we compute the other determinants
3 3 3
2 2 2
1 1 1
d b c
d b c
d b c
Dx
3 3 3
2 2 2
1 1 1
a d c
a d c
a d c
Dy
3 3 3
2 2 2
1 1 1
a b d
a b d
a b d
Dz
Now that we have that, we simply need to determine the solutions by the formulas
D
D
x
x D
D
y
y D
D
z
z
We get
x
y
z
So the solution to the system is (-2, 4, 1).
b. As we did in part a. above, we need to start by determining all of the determinants required for
Cramer’s Rule. We get
D
Dx
4 3 1
Dy
4 2 3
Dz
We will leave it to the reader to work through the details of these determinants. It can be shown
that these determinants give us
D 6 Dx 12 Dy 6 Dz 18
Plugging in gives us
D
D
x
x
D
D
y
y
D
D
z
z
So the solution to the system is (2, -1, -3).
The only real “hang-up” with Cramer’s Rule is the unfortunate situation in which the denominator
determinant ends up as zero. If this is the case, Cramer’s Rule does not work. You would have
to go back and solve the system using one of the other methods that you have learned for solving
a system.
13.3 Exercises
Write the coefficient matrix and the augmented matrix of the following systems.
2
x y
x y
2 6
x y
x y
3 6 4
x y
x y
2
17 2
7
z
y z
x y z
2 0
x y z
x y
x
4 7
x z
y
x z
Find the determinant of each matrix.
4 2 1
x y z
x y z
x y z
3 7 9 10
x y z
x y z
x y z
6 8 18 5
x y z
x y z
x y z
3
x y z
x y z
x y z
2 2 9
x y z
x y
x y z
2 10 8 8
x y z
x z
x y z
3 4 5 8
x y z
x y z
x y z
2 3 4
x y z
x y z
x y z
2 3 5 7
x y z
x y z
x y z
