# Solving Systems of Linear Equations in Three Variables - DOC - DOC

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```							                    Solving Systems of Linear Equations in Two Variables
A Brief Review with Examples

Recall from previous courses that a system of linear equations in two variables can be written in
the form
 a1 x  b1 y  c1
                  ,
a2 x  b2 y  c2

where an, bn, and cn are any real number.

Since there are two unknown quantities (variables), the system must consist of at least two
equations to find those unknown quantities. It is possible, however, to have more than two
equations. This often occurs when an application has several conditions for the unknown
variables.

Although the systems described here are linear (each equation in the system is linear), it is
possible to have non-linear equations in systems. These are referred to as systems of non-linear
equations in two variables, and can often be solved using techniques similar to those described
below.

To solve a system of equations in two variables means to find the coordinate pair (x, y) that
makes equations in the system a true statement. There are three possible situations when solving
systems, and they are described in the table below.

Type of Solution            Graphical Representation           Description of System
One Solution                Intersecting Lines             Consistent & Independent
Infinitely Many Solutions           Coinciding Lines              Consistent & Dependent
No Solution                  Parallel Lines                     Inconsistent

While there are several techniques for solving these systems of equations – the graphing,
substitution, and elimination methods are the most common – they each have there own
advantages and disadvantages. Regardless of the method used, you can always check solutions
by plugging the found x and y values into all equations in the system – if you get true statements
for all equations, you have a solution.

The following pages show one example of each type of solution. For each system, the solution is
found using the graphing, substitution, and elimination methods.
Example 1 – One Solution

2 x  y  2
Solve the system              .
 x  y  7

Graphing Method                  Substitution Method                Elimination Method
Graph each equation to find       Solve the first equation for y.    Add down to eliminate y and
the point of intersection.                                           solve for x.
y = 2x  2
2 x  y  2

Substitute this into the second             
equation.                                    x  y  7

3x  9
x – (2x  2) = 7                           x  3
Solve for x to get x = 3.         Multiply the second equation
by 2, then add down to
Substitute x = 3 into the first   eliminate x and solve for y.
The lines intersect at the        equation to get y.
coordinate (3, 4), so that is
 2 x  y  2

the solution to the system.             y = 2(3)  2 = 4                  
 2 x  2 y  14

The solution is (3, 4).                           3 y  12
y4

The solution is (3, 4).

Note: Once you find one value using the elimination method, it can often be quicker to
substitute that value into one of the original equations and solve for the remaining value.
Example 2 – Infinitely Many Solutions

 2x  4 y  2
Solve the system                .
 x  2 y  1

Graphing Method                    Substitution Method               Elimination Method
Graph each equation to find         Solve the second equation for     Multiply the second equation
the point of intersection.          x.                                by 2, then add down to
eliminate x.
x = 2y + 1
 2x  4 y  2

Substitute this into the first           
equation.                                 2 x  4 y  2

00
2(2y + 1) – 4y = 2
Since both variables were
Solve to get 2 = 2, a true        eliminated and a true
statement (or an identity).       statement resulted, there are
The lines are coinciding (they                                        infinitely many solutions to
are the same line when              Since the variable dropped        the system.
graphed), so there are              out and resulted in a true
infinitely many solutions to        statement, there are infinitely
the system.                         many solutions to the system.

Note: You can still check your work if a system has infinitely many solutions. Find two
coordinates that work in one equation and see if they also work in the other equation. If both
points do work in both equations, the only possibility is that there are infinitely many solutions.
Example 3 – No Solution

 3x  y  2
Solve the system             .
3x  y  0

Graphing Method                 Substitution Method             Elimination Method
Graph each equation to find      Solve the second equation for    Add down to eliminate x.
the point of intersection.       y.
 3x  y  2

y = 3x                      
 3x  y  0

Substitute this into the first                   02
equation.
Since both variables were
3x – (3x) = 2           eliminated and a false
statement resulted, there are
Solve to get 0 = 2, a false      no solutions to the system.
statement.
The lines are parallel and
never intersect, so there are    Since the variable dropped
no solutions to the system.      out and resulted in a false
statement, there are no
solutions to the system.

Note: Checking for no solution algebraically is not efficient. A quicker technique is to check the
slope of each linear equation (remember that m = a/b). If the slope is the same and you have
eliminated the possibility of coinciding lines, the only possibility is no solution.

```
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