7.2

Warm UpWarm Up

Lesson QuizLesson Quiz

Lesson PresentationLesson Presentation

Solve Linear Systems by Substitution

7.2 Warm-Up

Solve the equation.

1. 6a – 3 + 2a = 13 2. 4(n + 2) – n = 11

ANSWER a = 2 ANSWER n = 1

3. You burned 8 calories per minute on a treadmilland 10 calories per minute on an elliptical trainer for a total of 560 calories in 60 minutes. How many minutes did you spend on each machine?

ANSWERtreadmill: 20 min,elliptical trainer: 40 min

7.2 Example 1

Solve the linear system: y = 3x + 2

Equation 2

Equation 1

x + 2y = 11

Solve for y. Equation 1 is already solved for y.

SOLUTION

STEP 1

7.2 Example 1

7x + 4 = 11 Simplify.

7x = 7 Subtract 4 from each side.

x = 1 Divide each side by 7.

Substitute 3x + 2 for y.x + 2(3x + 2) = 11

Write Equation 2.x + 2y = 11

Substitute 3x + 2 for y in Equation 2 and solve for x.

STEP 2

7.2 Example 1

ANSWER

The solution is (1, 5).

Substitute 1 for x in the original Equation 1 to find the value of y.

y = 3x + 2 = 3(1) + 2 = 3 + 2 = 5

STEP 3

7.2 Example 1

CHECK

y = 3x + 2

5 = 3(1) + 2?

5 = 5

Substitute 1 for x and 5 for y in each of the original equations.

x + 2y = 11

1 + 2 (5) = 11?

11 = 11

7.2 Example 2

Solve the linear system: x – 2y = –6 Equation 1

4x + 6y = 4 Equation 2

SOLUTION

Solve Equation 1 for x.

x – 2y = –6 Write original Equation 1.

x = 2y – 6 Revised Equation 1

STEP 1

7.2 Example 2

Substitute 2y – 6 for x in Equation 2 and solve for y.

4x + 6y = 4 Write Equation 2.

4(2y – 6) + 6y = 4 Substitute 2y – 6 for x.

Distributive property8y – 24 + 6y = 4

14y – 24 = 4 Simplify.

14y = 28 Add 24 to each side.

y = 2 Divide each side by 14.

STEP 2

7.2 Example 2

Substitute 2 for y in the revised Equation 1 to find the value of x.

x = 2y – 6 Revised Equation 1

x = 2(2) – 6 Substitute 2 for y.

x = –2 Simplify.

ANSWER The solution is (–2, 2).

STEP 3

7.2 Example 2

CHECK

–2 – 2(2) = –6?

–6 = –6

Substitute –2 for x and 2 for y in each of the original equations.

4x + 6y = 4

4 = 4

Equation 1 Equation 2

x – 2y = –6

4(–2) + 6 (2) = 4 ?

7.2 Guided Practice

Solve the linear system using the substitution method.

3x + y = 10

y = 2x + 51. ANSWER (1, 7)

x + 2y = –6

x – y = 32.ANSWER (0, –3)

–2x + 4y = 0

3x + y = –73. ANSWER (–2, –1)

7.2 Example 3

Many businesses pay website hosting companies to store and maintain the computer files that make up their websites. Internet service providers also offer website hosting. The costs for website hosting offered by a website hosting company and an Internet service provider are shown in the table. Find the number of months after which the total cost for website hosting will be the same for both companies.

WEBSITES

7.2 Example 3

SOLUTION

Write a system of equations. Let y be the totalcost after x months.

Equation 1: Internet service provider

y = 10 + 21.95 x

STEP 1

7.2 Example 3

Equation 2: Website hosting company

y = 22.45 x

The system of equations is:

y = 22.45x

Equation 1y = 10 + 21.95x

Equation 2

7.2 Example 3

Substitute 22.45x for y in Equation 1 and solvefor x.

y = 10 + 21.95x

22.45x = 10 + 21.95x

0.5x = 10

x = 20

The total cost will be the same for both companies after 20 months.

ANSWER

STEP 2

Write Equation 1.

Substitute 22.45x for y.

Subtract 21.95x from each side.

Divide each side by 0.5.

7.2 Guided Practice

4. In Example 3, what is the total cost for website hosting for each company after 20 months?

$449ANSWER

5. WHAT IF? In Example 3, suppose the Internet service provider offers $5 off the set-up fee. After how many months will the total cost for website hosting be the same for both companies?

10 moANSWER

7.2 Example 4

For extremely cold temperatures, an automobile manufacturer recommends that a 70% antifreeze and 30% water mix be used in the cooling system of a car. How many quarts of pure (100%) antifreeze and a 50% antifreeze and 50% water mix should be combined to make 11 quarts of a 70% antifreeze and 30% water mix?

ANTIFREEZE

SOLUTION

Write an equation for the total number of quarts and an equation for the number of quarts of antifreeze. Let x be the number of quarts of 100% antifreeze, and let y be the number of quarts of a 50% antifreeze and 50% water mix.

STEP 1

7.2 Example 4

Equation 1: Total number of quarts

x + y = 11

Equation 2: Number of quarts of antifreeze

x quarts of100% antifreeze

y quarts of50%–50% mix

11 quarts of70%–30% mix

1 x + 0.5 y = 0.7(11)

x + 0.5y = 7.7

7.2 Example 4

The system of equations is:

x + 0.5y = 7.7

Solve Equation 1 for x.x + y = 11

x = 11 – y

Substitute 11 – y for x in Equation 2 and solvefor y.

x + 0.5y = 7.7

STEP 2

STEP 3

Equation 1x + y =11

Equation 2

Write Equation 1

Revised Equation 1

Write Equation 2.

7.2 Example 4

(11 – y) + 0.5y = 7.7

y = 6.6

Substitute 6.6 for y in the revised Equation 1 tofind the value of x.

STEP 4

x = 11 – y = 11 – 6.6 = 4.4

ANSWER

Mix 4.4 quarts of 100% antifreeze and 6.6 quarts of a 50%antifreeze and 50% water mix to get 11 quarts of a 70%antifreeze and 30% water mix.

Substitute 11 – y for x.

Solve for y.

7.2 Guided Practice

WHAT IF? How many quarts of 100% antifreeze and a 50% antifreeze and 50% water mix should be combined to make 16 quarts of a 70% antifreeze and 30% water mix?

6.

ANSWER

6.4 quarts of 100% antifreeze and 9.6 quarts of a 50%antifreeze and 50% water mix

7.2 Lesson Quiz

Solve the linear system using substitution

1. –5x – y = 12

3x – 5y = 4

ANSWER (–2, –2)

2. 2x + 9y = –4

x – 2y = 11

ANSWER (7, –2)

7.2 Lesson Quiz

3. You are making 6 quarts of fruit punch for a party. You want the punch to contain 80% fruit juice. You have bottles of 100% fruit juice and 20% fruit juice. How many quarts of 100% fruit juice and how many quarts of 20% fruit juice should you mix to make 6 quarts of 80% fruit juice?

ANSWER

4.5 quarts of 100% fruit juice and 1.5 quarts of 20% fruit juice