This document provides examples for solving systems of linear equations by graphing. It begins by defining key terms like systems of linear equations and their solutions. Examples are then given of identifying whether an ordered pair is a solution by substituting into the equations. The document explains that solutions are found at the intersection point of the graphs. Two examples graph systems and find the solution. The document ends with a word problem about two girls reading pages from a book, which is modeled with a system of equations and solved graphically.

1. 6-1 Solving Systems by Graphing Lesson Presentation

2. Identify solutions of linear equations in two variables. Solve systems of linear equations in two variables by graphing. Objectives

3. systems of linear equations solution of a system of linear equations Vocabulary

4. A system of linear equations is a set of two or more linear equations containing two or more variables. A solution of a system of linear equations with two variables is an ordered pair that satisfies each equation in the system. So, if an ordered pair is a solution, it will make both equations true.

5. Tell whether the ordered pair is a solution of the given system. Example 1A: Identifying Systems of Solutions (5, 2); The ordered pair (5, 2) makes both equations true. (5, 2) is the solution of the system. Substitute 5 for x and 2 for y in each equation in the system. 3 x – y = 13 2 – 2 0 0 0  0 3 (5) – 2 13 15 – 2 13 13 13  3 x – y 13

6. If an ordered pair does not satisfy the first equation in the system, there is no reason to check the other equations. Helpful Hint

7. Example 1B: Identifying Systems of Solutions Tell whether the ordered pair is a solution of the given system. (–2, 2); x + 3 y = 4 – x + y = 2 Substitute –2 for x and 2 for y in each equation in the system. The ordered pair (–2, 2) makes one equation true but not the other. (–2, 2) is not a solution of the system.  – 2 + 3 (2) 4 x + 3 y = 4 – 2 + 6 4 4 4 – x + y = 2 – (–2) + 2 2 4 2

8. All solutions of a linear equation are on its graph. To find a solution of a system of linear equations, you need a point that each line has in common. In other words, you need their point of intersection. The point (2, 3) is where the two lines intersect and is a solution of both equations, so (2, 3) is the solution of the systems. y = 2 x – 1 y = – x + 5

9. Sometimes it is difficult to tell exactly where the lines cross when you solve by graphing. It is good to confirm your answer by substituting it into both equations. Helpful Hint

10. Solve the system by graphing. Check your answer. Example 2A: Solving a System Equations by Graphing y = x y = –2 x – 3 Graph the system. The solution appears to be at (–1, –1). (–1, –1) is the solution of the system. y = x y = –2 x – 3 • (–1, –1) Check Substitute (–1, –1) into the system. y = x (–1) (–1) – 1 –1  y = –2 x – 3 ( – 1) –2 ( – 1) –3 – 1 2 – 3 – 1 – 1 

11. Solve the system by graphing. Check your answer. Example 2B: Solving a System Equations by Graphing y = x – 6 Rewrite the second equation in slope-intercept form. Graph using a calculator and then use the intercept command. y + x = –1 y = x – 6 y + x = –1 − x − x y =

12. Solve the system by graphing. Check your answer. Example 2B Continued Check Substitute into the system. y = x – 6 The solution is . + – 1 – 1 – 1 – 1 – 1  y = x – 6 – 6 

13. Example 3: Problem-Solving Application Wren and Jenni are reading the same book. Wren is on page 14 and reads 2 pages every night. Jenni is on page 6 and reads 3 pages every night. After how many nights will they have read the same number of pages? How many pages will that be?

14. The answer will be the number of nights it takes for the number of pages read to be the same for both girls. List the important information: Wren on page 14 Reads 2 pages a night Jenni on page 6 Reads 3 pages a night Example 3 Continued 1 Understand the Problem

15. Write a system of equations, one equation to represent the number of pages read by each girl. Let x be the number of nights and y be the total pages read. Example 3 Continued 2 Make a Plan Total pages is number read every night plus already read. Wren y = 2  x + 14 Jenni y = 3  x + 6

16. Example 3 Continued Graph y = 2 x + 14 and y = 3 x + 6 . The lines appear to intersect at (8, 30). So, the number of pages read will be the same at 8 nights with a total of 30 pages. Solve 3  (8, 30) Nights

17. Check ( 8 , 30 ) using both equations. Number of days for Wren to read 30 pages. Number of days for Jenni to read 30 pages. Example 3 Continued Look Back 4 3 (8) + 6 = 24 + 6 = 30  2 (8) + 14 = 16 + 14 = 30 

18. Check It Out! Example 3 Video club A charges $10 for membership and $3 per movie rental. Video club B charges $15 for membership and $2 per movie rental. For how many movie rentals will the cost be the same at both video clubs? What is that cost?

20. Write a system of equations, one equation to represent the cost of Club A and one for Club B. Let x be the number of movies rented and y the total cost. Check It Out! Example 3 Continued 2 Make a Plan Total cost is price for each rental plus member- ship fee. Club A y = 3  x + 10 Club B y = 2  x + 15

21. Graph y = 3 x + 10 and y = 2 x + 15. The lines appear to intersect at (5, 25). So, the cost will be the same for 5 rentals and the total cost will be $25. Check It Out! Example 3 Continued Solve 3

22. Check ( 5 , 25 ) using both equations. Number of movie rentals for Club A to reach $25: Number of movie rentals for Club B to reach $25: Check It Out! Example 3 Continued Look Back 4 2 (5) + 15 = 10 + 15 = 25  3 (5) + 10 = 15 + 10 = 25 