Linear
Inequalities
in
2-5
2-5 Linear Inequalities in Two Variables
Two Variables
LESSON PLAN
1. Warm Up
(Slide #2)
2. Objective and PA State Standards
3. Vocab
(Slides #4 - 5)
4. Lesson Presentation
5. Text Questions
(Slide #6 – 33)
(Slide #34)
6. Worksheet 2.5B
(Slide #35)
7. Lesson Quiz
Holt
Algebra
Holt
Algebra
22
(Slide #3)
(Slide #36,37)
2-5
Linear Inequalities in Two Variables
Warm Up
Find the intercepts of each line.
1. 3x + 2y = 18
2. 4x – y = 8
3. 5x + 10 = 2y
Write the function in slope-intercept form.
Then graph.
4. 2x + 3y = –3
Holt Algebra 2
2-5
Linear Inequalities in Two Variables
Objectives
1. Graph linear inequalities on the coordinate plane
2. Solve problems using linear inequalities
PA Dept. of Educ. Math Standards and Anchors:
2.8.A2.B,F
M11.D.2.1.5
Holt Algebra 2
2-5
Linear Inequalities in Two Variables
Vocabulary
linear inequality
Holt Algebra 2
2-5
Linear Inequalities in Two Variables
A linear inequality relates two variables using an
inequality symbol such as y > 2x – 4.
Its graph is a region (…or a wedge-shaped area) of the
coordinate plane bounded by a line.
Holt Algebra 2
2-5
Linear Inequalities in Two Variables
For example:
Holt Algebra 2
2-5
Linear Inequalities in Two Variables
Helpful Hint
Think of the underlines in the symbols ≤ and ≥
as representing solid lines on the graph.
Holt Algebra 2
2-5
Linear Inequalities in Two Variables
Example 1A: Graphing Linear Inequalities
Graph the inequality
The boundary
.
has a y-int. +2 and slope of
Draw the line dashed since it is not
part of the solution.
Shade the region above the
boundary line to show
.
Holt Algebra 2
.
2-5
Linear Inequalities in Two Variables
Example 1A Continued
Check Choose a point in the solution region, such
as (3, 2) and test it in the inequality.
?
?
2>1
The test point satisfies the
inequality, so the solution
region appears to be correct.
Holt Algebra 2
2-5
Linear Inequalities in Two Variables
Example 1B: Graphing Linear Inequalities
Graph the inequality y ≤ –1.
Recall that y= –1 is a
horizontal line.
Step 1 Draw a solid line for
y=–1 because the boundary
line is part of the graph.
Step 2 Shade the region
below the boundary line to
show where y < –1.
Holt Algebra 2
2-5
Linear Inequalities in Two Variables
Example 1B Continued
Check The point (0, –2) is a
solution because –2 ≤ –1.
Note that any point on or
below y = –1 is a solution,
regardless of the value of x.
Holt Algebra 2
2-5
Linear Inequalities in Two Variables
Check It Out! Example 1a
Graph the inequality y ≥ 3x –2.
The boundary line has a y–intercept of ____
-2
and a slope of _____.
+3
solid line because
Draw a ______
it is part of the solution.
Then shade the region
above the boundary line
_______
to show y > 3x – 2.
Holt Algebra 2
2-5
Linear Inequalities in Two Variables
Check It Out! Example 1a Continued
Check Is (–3, 2) a solution to the inequality?
y ≥ 3x –2
?
2 ≥ 3(–3) –2
?
2 ≥ (–9) –2
?
2 > –11 
The test point satisfies the
inequality, so the solution
region appears to be correct.
Holt Algebra 2
2-5
Linear Inequalities in Two Variables
Check It Out! Example 1b
Graph the inequality y < –3.
Recall that y = –3 is a
horizontal
________________line.
Step 1 Draw the boundary
dashed because it is
line _________
not part of the solution.
Step 2 Shade the region
below
_________
the boundary
line to show where y < –3.
Holt Algebra 2
2-5
Linear Inequalities in Two Variables
Check It Out! Example 1b Continued
Check Is (0, –4) a solution? Yes!
Holt Algebra 2
2-5
Linear Inequalities in Two Variables
Helpful Hint
The point (0, 0) is the easiest point to test if it is
not on the boundary line.
Holt Algebra 2
2-5
Linear Inequalities in Two Variables
Example 2: Graphing Linear Inequalities Using
Intercepts
Graph 3x + 4y ≤ 12 using intercepts.
Step 1 Find the intercepts.
y-intercept (is where x = 0)
x-intercept (is where y = 0)
3x + 4y = 12
3(0) + 4y = 12
3x + 4y = 12
3x + 4(0) = 12
4y = 12
y=3
3x = 12
x=4
Holt Algebra 2
2-5
Linear Inequalities in Two Variables
Example 2 Continued
Graphing 3x + 4y ≤ 12 using intercepts:
Step 2 Draw the boundary line.
The line goes through (0, 3) and (4, 0).
Draw a solid line for the boundary line
because it is part of the graph.
(0, 3)
Step 3 Find the correct region
to shade.
Substitute (0, 0) into the inequality.
Because 0 + 0 ≤ 12 is true, shade the
region that contains (0, 0).
Holt Algebra 2
(4, 0)
2-5
Linear Inequalities in Two Variables
Check It Out! Example 2
Graph 3x – 4y > 12 using intercepts.
Step 1 Find the intercepts.
Substitute x = 0 and y = 0 into 3x – 4y = 12 to
find the intercepts of the boundary line.
y-intercept
3x – 4y = 12
3(0) – 4y = 12
x-intercept
3x – 4y = 12
3x – 4(0) = 12
– 4y = 12
y=–3
3x = 12
x=4
Holt Algebra 2
2-5
Linear Inequalities in Two Variables
Check It Out! Example 2
Graph 3x – 4y > 12 using intercepts.
Step 2 Draw the boundary line.
The line goes through (0, –3) and (4, 0).
Draw the boundary line dashed because
it is not part of the solution.
(4, 0)
Step 3 Find the correct region
to shade.
Substitute (0, 0) into the inequality.
Because 0 + 0 >12 is false, shade the
region that does not contain (0, 0).
Holt Algebra 2
(0, –3)
2-5
Linear Inequalities in Two Variables
Many applications of inequalities in two variables
use only nonnegative values for the variables.
Graph only the part of the plane that includes
realistic solutions.
Caution
Don’t forget which variable represents which
quantity.
Holt Algebra 2
2-5
Linear Inequalities in Two Variables
Example 3: Problem-Solving Application
A school carnival charges $4.50 for
adults and $3.00 for children. The
school needs to make at least $135 to
cover expenses.
A. Using x as the number of adult tickets and
y as the number of child tickets, write and
graph an inequality for the amount the
school makes on ticket sales.
B. If 25 child tickets are sold, how many adult
tickets must be sold to cover expenses?
Holt Algebra 2
2-5
Linear Inequalities in Two Variables
Let x represent the number of adult tickets
and y represent the number of child tickets
that must be sold. Write an inequality to
represent the situation.
Adult
price
4.50
times
number of
adult
tickets
•
x
plus
child
price
+
3.00
times
number
of child
tickets
is at
least
total.
•
y

135
An inequality that models the problem is 4.5x + 3y ≥ 135.
Holt Algebra 2
2-5
Linear Inequalities in Two Variables
4.5x + 3y ≥ 135.
Find the intercepts of the boundary line.
4.5(0) + 3y = 135
4.5x + 3(0) = 135
y = 45
Graph the boundary line
through (0, 45) and (30, 0)
as a solid line.
Shade the region above the
line that is in the first
quadrant, as ticket sales
cannot be negative.
Holt Algebra 2
x = 30
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Linear Inequalities in Two Variables
If 25 child tickets are sold,
4.5x + 3(25) ≥ 135
4.5x + 75 ≥ 135
_
4.5x ≥ 60, so x ≥ 13.3
At least 14 adult tickets must be sold.
Holt Algebra 2
2-5
Linear Inequalities in Two Variables
Check It Out! Example 3
A café gives away prizes. A large prize
costs the café $125, and the small
prize costs $40. The café will not spend
more than $1500. How many of each
prize can be awarded? How many small
prizes can be awarded if 4 large prizes
are given away?
Holt Algebra 2
2-5
Linear Inequalities in Two Variables
Let x represent the number of small prizes and y
represent the number of large prizes, the total not
too exceed $1500. Write an inequality to represent
the situation.
Small
prize
times
40
number
awarded
plus
large
prize
x
+
125
times
number
awarded
is less
than
total.
y
≤
1500
An inequality that models the problem is 40x + 125y ≤ 1500
Holt Algebra 2
2-5
Linear Inequalities in Two Variables
40x + 125y ≤ 1500
Find the intercepts of the boundary line.
40(0) + 125y = 1500
y = 12
40x + 125(0) = 1500
x = 37.5
Graph the boundary line
through (0, 12) and (37.5, 0)
as a solid line.
Shade the region below the
line that is in the first
quadrant, as prizes awarded
cannot be negative.
Holt Algebra 2
2-5
Linear Inequalities in Two Variables
40x + 125y ≤ 1500
If 4 large prizes are awarded,
40x + 125(4) ≤ 1500
40x + 500 ≤ 1500
40x ≤ 1000
x ≤ 25
No more than 25 small prizes can be awarded.
Holt Algebra 2
2-5
Linear Inequalities in Two Variables
Graphing a linear inequality with a graphing calculator.
Press the
key.
Press the left arrow key to move to the left side of
Each time you press
styles shown here.
>
<
Holt Algebra 2
you will see one of the graph
2-5
Linear Inequalities in Two Variables
Check It Out! Example 4 Continued
Use the calculator graph the solution to y < 4x - 4
1.
2.
3.
4.
5.
6.
Key “Y=“
Key “<“ twice
Key “ENTER” three times to get to the “less than” graphic
Key “>” twice to return to the equation area
Key “4X-4”
Key “GRAPH”
Holt Algebra 2
2-5
Linear Inequalities in Two Variables
Check It Out! Example 4 Continued
Use the calculator graph the solution to
1.
2.
3.
4.
5.
6.
Key “Y=“
Key “<“ twice
Key “ENTER” three times
Key “>” twice
Key “(3÷4)X-3”
Key “GRAPH”
Holt Algebra 2
2-5
Linear Inequalities in Two Variables
Graph this inequality on a calculator: 2x + 3y < -5
To use the graphing calculator, you must first
rewrite the inequality so you are “solving for ‘y’”)
2x + 3y < -5
-2x
-2x
3y < -2x – 5
/3
/3
/3
y < -(2/3)x - 5/3
Holt Algebra 2
2-5
Linear Inequalities in Two Variables
p.128 #2 - 20 EVEN
Holt Algebra 2
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Linear Inequalities in Two Variables
Holt Algebra 2
2-5
Linear Inequalities in Two Variables
Lesson Quiz: Part I
1. Graph 2x –5y  10
using intercepts.
Holt Algebra 2
2. Solve –6y < 18x – 12 for y.
Graph the solution.
2-5
Linear Inequalities in Two Variables
Lesson Quiz: Part II
3. Potatoes cost a chef $18 a box, and carrots cost
$12 a box. The chef wants to spend no more
than $144. Use x as the number of boxes of
potatoes and y as the number of boxes of
carrots.
a. Write an inequality for the number of boxes
the chef can buy.
b. How many boxes of potatoes can the chef
order if she orders 4 boxes of carrot?
Holt Algebra 2