SOLVING SYSTEMS OF LINEAR EQUATIONS BY GRAPHING

Example 1 :

Solve the following system of equations by graphing.

x + y - 4  =  0

3x - y  =  0

Solution :

Step 1 :

Let us re-write the given equations in slope-intercept form  (y = mx + b).

y  =  - x + 4

(slope is -1 and y-intercept is 4)

y  =  3x

(slope is 3 and y-intercept is 0)

Based on slope and y-intercept, we can graph the given equations.

Step 2 :

Find the point of intersection of the two lines. It appears to be (1, 3). Substitute to check if it is a solution of both equations.

Because the point (1, 3) satisfies both the equations, the solution of the system is (1, 3).  

Example 2 :

Solve the following system of equations by graphing.

3x - y - 3  =  0 

x - y - 3  =  0

Solution :

Step 1 :

Let us re-write the given equations in slope-intercept form. 

y  =  3x - 3

(slope is 3 and y-intercept is -3) 

y  =  x - 3 

(slope is 1 and y-intercept is -3) 

Based on slope and y-intercept, we can graph the given equations. 

Step 2 :

Find the point of intersection of the two lines. It appears to be (0, -3). Substitute to check if it is a solution of both equations.

Because the point (0, -3) satisfies both the equations, the solution of the system is (0, -3).  

Example 3 :

During school vacation, Alex wants to go bowling and to play laser tag. He wants to play 6 total games but needs to figure out how many of each he can play if he spends exactly $20. Each game of bowling is $2 and each game of laser tag is $4.

Solution :

Step 1 :

Let "x" be the no. of games Alex bowls and "y" be the no. of laser tag he plays. 

No. of games he bowls + No. of laser tags  =  Total games

x + y  =  6 -------- (1)

Step 2 : 

Write an equation which represents the money spent on these two items.

Cost of "x" no. of games he bowls  =  2x

Cost of "y" no. of laser tags he plays  =  4y

Total cost  =  $20

Then, we have 

2x + 4y  =  20

Divide both sides by 2.

x + 2y  =  10 -------- (2)

Step 3 :

Write the equations (1) and (2) in slope-intercept form. Then graph.

(1) ----- > x + y  =  6 ----- > y  =  -x + 6 

(2) ---- > x + 2y  =  10 --- > y  =  -(1/2)x + 5 

Graph the equations y = -x + 6 and y = -(1/2)x + 5.

Step 4 :

Look at the graph and identify the solution of the system of equations. Check your answer by substituting the ordered pair into both equations.

The point of intersection of the lines is (2, 4). 

Let us check whether the ordered pair (2, 4) satisfies both the equations. 

Since the ordered pair (2, 4) satisfies both the equations, the solution of the system is (2, 4). 

Step 5 :

Interpret the solution in the original context.

Alex will bowl 2 games and play 4 games of laser tag.

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