1. Solving Systems of Linear Equations by Graphing
4.1
Solving Systems of Linear Equations by Graphing

2. Systems of Linear Equations
A system of linear equations consists of two or more linear equations.
This section focuses on solving systems of linear equations containing two equations in two variables.
A solution of a system of linear equations in two variables is an ordered pair of numbers that is a solution of both equations in the system.

3. Example Determine whether (–3, 1) is a solution of the system
x – y = – 4
2x + 10y = 4
Replace x with –3 and y with 1 in both equations.
First equation: –3 – 1 = – True
Second equation: 2(–3) + 10(1) = – = True
Since the point (–3, 1) produces a true statement in both equations, it is a solution of the system.

4. Example Determine whether (4, 2) is a solution of the system
2x – 5y = – 2
3x + 4y = 4
Replace x with 4 and y with 2 in both equations.
First equation: 2(4) – 5(2) = 8 – 10 = – True
Second equation: 3(4) + 4(2) = = 20 ≠ False
Since the point (4, 2) produces a true statement in only one equation, it is NOT a solution.

5. Solving Systems of Equations by Graphing
Since a solution of a system of equations is a solution common to both equations, it is also a point common to the graphs of both equations.
To find the solution of a system of two linear equations, we graph the equations and see where the lines intersect.

6. Example Solve the system of equations by graphing. 2x – y = 6
(-5, 5)
(6, 6)
(-2, 4)
(4, 2)
(1, 3)
First, graph 2x – y = 6.
(3, 0)
Second, graph x + 3y = 10.
The lines APPEAR to intersect at (4, 2).
(0, -6)
continued

7. continued
Although the solution to the system of equations appears to be (4, 2), you still need to check the answer by substituting x = 4 and y = 2 into the two equations.
First equation:
2(4) – 2 = 8 – 2 = True
Second equation:
4 + 3(2) = = True
The point (4, 2) checks, so it is the solution of the system.

8. Helpful Hint
Neatly drawn graphs can help when “guessing” the solution of a system of linear equations by graphing.

9. Example Solve the system of equations by graphing. –x + 3y = 6
(6, 4)
(0, 2)
(6, 1)
(3, 0)
First, graph – x + 3y = 6.
(-6, 0)
(0, -1)
Second, graph 3x – 9y = 9.
The lines APPEAR to be parallel.
continued

10. continued
Although the lines appear to be parallel, we need to check their slopes.
–x + 3y = First equation
3y = x Add x to both sides.
y = x Divide both sides by 3.
3x – 9y = Second equation
–9y = –3x Subtract 3x from both sides.
y = x – Divide both sides by –9.
Both lines have a slope of , so they are parallel and do not intersect. Hence, there is no solution to the system.

11. Example Solve the system of equations by graphing. x = 3y – 1
(5, 2)
(-1, 0)
(2, 1)
First, graph x = 3y – 1.
(-4, -1)
(7, -2)
Second, graph 2x – 6y = –2.
The lines APPEAR to be identical.
continued

12. continued
Although the lines appear to be identical, we need to check that their slopes and y-intercepts are the same.
x = 3y – 1 First equation
3y = x Add 1 to both sides.
y = x Divide both sides by 3.
2x – 6y = – Second equation
–6y = – 2x – Subtract 2x from both sides.
y = x Divide both sides by -6.
Any ordered pair that is a solution of one equation is a solution of the other. This means that the system has an infinite number of solutions.

13. Identifying Special Systems of Linear Equations
There are three possible outcomes when graphing two linear equations in a plane.
One point of intersection—one solution
Parallel lines—no solution
Coincident lines—infinite number of solutions
If there is at least one solution, the system is considered to be consistent.
If the system defines distinct lines, the equations are independent.

14. Possible Solutions of Linear Equations
Graph
Type of System
Number of Solutions
If the lines intersect, the system of equations has one solution given by the point of intersection.
Consistent
The equations are independent.
(3, 5)
Two lines intersect at one point.
If the lines are parallel, then the system of equations has no solution because the lines never intersect.
Inconsistent
The equations are independent.
Parallel lines
If the lines lie on top of each other, then the system has infinitely many solutions. The solution set is the set of all points on the line.
Consistent
The equations are dependent.
Lines coincide

15. Identifying Special Systems of Linear Equations
Since there are only three possible outcomes with two lines in a plane, we can determine how many solutions of the system there will be without graphing the lines.
Change both linear equations into slope-intercept form.
We can then easily determine if the lines intersect, are parallel, or are the same line.

16. Example
Without graphing, determine the number of solutions of the system. 3x + y = 1
3x + 2y = 6
Write each equation in slope-intercept form.
3x + y = 1 First equation
y = –3x Subtract 3x from both sides.
3x + 2y = 6 Second equation
2y = –3x Subtract 3x from both sides.
Divide both sides by 2.
The lines are intersecting lines (since they have different slopes), so the system is consistent and independent and has one solution.

17. Example
Without graphing, determine the number of solutions of the system.
3x + y = 0
2y = –6x
Write each equation in slope-intercept form.
3x + y = 0 First equation
y = –3x Subtract 3x from both sides.
2y = –6x Second equation
y = –3x Divide both sides by 2.
The two lines are identical, so the system is consistent and dependent, and there are infinitely many solutions.

18. Example
Without graphing, determine the number of solutions of the system.
2x + y = 0
y = –2x + 1
Write each equation in slope-intercept form.
2x + y = 0 First equation
y = –2x Subtract 2x from both sides.
y = –2x + 1 Second equation is already in
slope-intercept form.
The two lines are parallel lines. Therefore, the system is inconsistent and independent, and there are no solutions.