The Graph of Ax + By = C 163
Lesson
Lesson 3-3
3-3
Mental Math
Give a general variation
equation based on the
description.
a. The cost c of painting
the interior of a house
varies directly as the
number n of rooms to
be painted.
b. The amount p of paint
needed for a wall varies
jointly as the length and
height h of the wall.
c. The time t it will take
to paint varies inversely
as the number n of
painters hired.
d. The time d needed for
the paint to dry varies
directly as the thickness t
of the paint applied and
inversely as the square of
the amount a of air
circulation in the room.
BIG IDEA If a linear combination of two variables x and y is
a constant, then the graph of all the points (x, y) is a line.
Recall the equation 2.5x + 2y = 30
from the previous lesson. This equation
represents allowable $30 purchases of
x hamburgers at $2.50 each and y hot
dogs at $2.00 each from Harry’s
Hamburger Hovel. Because you do not
buy fractions of sandwiches, both x and
y are nonnegative integers. So a graph
of the solution is a set of discrete points.
However, if you allow x and y to be any
real numbers, then the graph of
2.5x + 2y = 30 is shown at the right.
The equation 2.5x + 2y = 30 is of the form Ax +
B
y =
C
, with A = 2.5,
B
= 2, and
C
= 30. When A and B are not both zero, the graph of
A
x +
B
y =
C
is always a line.
Standard Form of an Equation of a Line Theorem
The graph of Ax + By = C, where A and B are not both zero,
is a line.
Proof There are two cases to consider: (1) if B = 0 and (2) if B ≠ 0.
(1) If B = 0, then A ≠ 0, and the equation is simply Ax = C.
Multiply both sides by
1
__
A
to obtain the equivalent equation x =
C
__
A
.
The graph of this equation is a vertical line.
(2) If B ≠ 0, then solve the given equation for y:
Ax + By = C Given
By =
–
Ax + C Add
–
Ax to both sides.
y =
–
A
__
B
x +
C
__
B
Divide both sides by B.
x
y
2010 15
10
5
5
15
20
25
30
᎑ 15
᎑ 10᎑ 15᎑ 20
᎑ 10
᎑ 5
᎑ 5
2.5x
+
2y
=
30
(0, 15)
(4, 10)
(8, 5)
(12, 0)
x
y
2010 15
10
5
5
15
20
25
30
᎑ 15
᎑ 10᎑ 15᎑ 20
᎑ 10
᎑ 5
᎑ 5
2.5x
+
2y
=
30
(0, 15)
(4, 10)
(8, 5)
(12, 0)
The Graph of
Ax + By = C
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164 Linear Functions and Sequences
Chapter 3
QY1
Graphing a Line Using Intercepts
Because the form Ax + By = C can describe any line, it is called the
standard form of an equation for a line
. Although, if B ≠ 0, you
could rewrite such an equation in slope-intercept form in order to
make a graph, it is often much quicker to graph such equations by
hand using x- and y-intercepts. If
A
,
B
, and
C
are all nonzero, the line
with equation Ax + By = C has distinct x- and y-intercepts, so the
intercepts can be used to graph the line.
Example 1
Graph the equation
–
5x
+
2y = 10 using its x- and y-intercepts.
Solution
To nd the x-intercept, substitute 0 for y, and solve for x.
–
5x
+
2(0)
=
10
x
=
–2
The x-intercept is –2.
To nd the y-intercept, substitute 0 for x, and solve for y.
–5(0)
+
2y
=
10
y
=
5
The y-intercept is 5.
Plot (
–
2, 0) and (0, 5) and draw the line containing them, as shown at
the right.
Check
Find a third ordered pair that satis es
–
5x
+
2y = 10. For example,
when x = 2,
–
10 + 2y = 10, so 2y = 20 and y = 10. Thus, (2, 10) should
be on the graph. Is it? Yes. It checks.
This technique does not work when
A
,
B
, or C is zero. If A = 0, the
slope is
–
0
__
B
= 0. The line is horizontal, and so there is no x-intercept.
If B = 0, the slope of the line is
–
A
__
0
, which is undefi ned. The line is
vertical, and so there is no y-intercept.
QY2
QY1
When B ≠ 0, what are
the slope and
y-intercept
of the line with equation
Ax
+
By
=
C?
QY1
When B ≠ 0, what are
the slope and
y-intercept
of the line with equation
Ax
+
By
=
C?
y
x
10
15
2
5
04
᎑ 5
᎑ 4 ᎑ 2
(0, 5)
(᎑ 2, 0)
᎑ 5x
+
2y
=
10
y
x
10
15
2
5
04
᎑ 5
᎑ 4 ᎑ 2
(0, 5)
(᎑ 2, 0)
᎑ 5x
+
2y
=
10
QY2
Why can you not use the
x- and y-intercepts to
graph
Ax + By = C when
C = 0?
QY2
Why can you not use the
x- and y-intercepts to
graph
Ax + By = C when
C = 0?
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The Graph of Ax + By = C 165
Lesson 3-3
Example 2
Graph x + 0y = 3.
a. Is this the graph of a line?
b. Is this the graph of a function?
Solution
a. The equation simpli es to x = 3. The value of x is always 3, regardless of
the value of y.
The graph is a vertical line.
b. Create a table of values.
It is not the graph of a function because
more than one ordered pair has the same
x-coordinate.
Equivalent Equations for Lines
One drawback of the standard form is that the same line can have
many different, but equivalent, equations in standard form. Recall
that multiplying both sides of an equation by a nonzero real number
yields an equivalent equation. Since lines have a unique equation
in slope-intercept form, you can test equations for equivalence by
putting them in slope-intercept form.
Example 3
Find which equations below, if any, represent the same line.
(1) 4x + 1.5y = 12 (2) 8x + 3y = 24
(3) 8x + 3y = 12 (4) 16x + 6y = 12
Solution 1
Rewrite each line in slope-intercept form.
(1)
y
=
?
(2) y
=
?
(3) y
=
?
(4) y
=
?
Equations
?
are equivalent. Equations
?
are not
equivalent to any other given equations
.
Solution 2
If I multiply both sides of Equation
?
by 2,
Equation
?
results. So Equations
?
and
?
are equivalent.
Since the right side of three of the given equations is 12, no
other equations are equivalent.
y
x
2 4
4
2
᎑ 4
᎑ 2
᎑ 4 ᎑ 2
x
=
3
y
x
2 4
4
2
᎑ 4
᎑ 2
᎑ 4 ᎑ 2
x
=
3
xy
3 –5
30
32
xy
3 –5
30
32
GUIDEDGUIDED
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166 Linear Functions and Sequences
Chapter 3
Equations (2), (3), and (4) from Example 3 represent lines with the
same slope but different y-intercepts. This suggests that the graphs of
Ax +
B
y = C and Ax +
B
y =
D
are distinct parallel lines when C ≠
D
.
Questions
COVERING THE IDEAS
1. a.
Fill in the Blank If A and B are not both 0, the graph of
Ax +
By
= C is a
?
.
b. If A ≠ 0 but B = 0, what kind of line is the graph?
c. If B ≠ 0 but A = 0, what kind of line is the graph?
2. Fill in the Blank
A
x +
B
y = C is in the
?
of an equation
of a line.
3. What is true about the slope of a vertical line?
4. True or False Every line in standard form can be graphed by
drawing the line containing its x- and y-intercepts.
In 5 and 6, an equation for a line is given.
a. Find its x-intercept.
b. Find its y-intercept.
c. Graph the line using your answers to Parts a and b.
5. 4x + 9y = 36 6. 4x – 5y = 10
7. Consider Ax +
B
y =
C
.
a. Find the x-intercept of the line. What happens when A = 0?
b. Find the y-intercept of the line. What happens when B = 0?
APPLYING THE MATHEMATICS
8.
Write an equation in standard form for the line graphed at the
right.
9. Find the value of C such that the point (4,
–
1) lies on the graph
of 10x - 2y =
C
.
10. Delaney’s Deli makes ham and cheese sandwiches and turkey
sandwiches. Each ham and cheese sandwich uses
1
__
8
lb of cheese,
while each turkey sandwich uses no cheese. Let x be the number
of ham and cheese sandwiches the deli prepares. Let y be the
number of turkey sandwiches the deli prepares.
a. Write an equation stating that the total amount of cheese the
deli uses is 5 lb.
b. Graph your equation from Part a.
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The Graph of Ax + By = C 167
Lesson 3-3
11. a. Graph the line with equation 0x + 4y = 14.
b. Find two ordered pairs satisfying the equation.
c. Compute the slope of the line through the two points.
In 12 and 13, nd an equation for a line in standard form with the
given properties.
12. y-intercept
–
3 and slope 2
13. no y-intercept and passes through the point (17, 29.93)
14. Mallory combines
n
liters of a solution that is 3 mol/L chlorine
with y liters of a solution that is 5 mol/L chlorine. She ends up
with a mixture that contains 2 moles of chlorine.
a. Write an equation relating
n
, y, and the total amount of
chlorine in the mixture.
b. Graph the equation you obtained in Part a by fi nding the
n
- and y-intercepts. Consider
n
to be the independent
variable.
c. Use your graph to fi nd about how many liters of the 5 mol/L
solution Mallory must add to 0.4 liter of the 3 mol/L solution
to get the fi nal mixture.
15. Consider the graphs of Ax +
B
y = C and Ax +
B
y =
D
if A and
B are not both zero and C ≠
D
.
a. Rewrite each equation in slope-intercept form.
b. What is the relationship between the slopes of the lines?
What does that tell you about the lines?
c. What is the relationship between the y-intercepts of the lines?
What does that tell you about the lines?
d. Write the conclusions of Parts b and c as one
if-then statement.
e. Use the if-then statement from Part d to give the equations of
several lines parallel to 16x – 13y = 11.
16. Use a CAS expand command to show that multiplying the
equation Ax +
B
y = C by a nonzero number
k
yields another
equation in standard form. Why must this new equation describe
the same line as the original?
REVIEW
17.
At a library book sale, paperbacks are being sold for $0.50 each
and hardcover books are $1. If you want to buy
P
paperbacks
and
H
hardcover books, write a linear combination that
expresses the amount you will have to pay.
(Lesson 3-2)
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168 Linear Functions and Sequences
Chapter 3
18. Suppose your car uses 0.035 gallon of gas to travel 1 mile
in the city and 0.027 gallon of gas to travel 1 mile on the
highway.
(Lesson 3-2)
a. Write a linear combination to express the number of gallons
of gas you would use to travel
C
miles in the city and
H
miles
on the highway.
b. If your car has 14 gallons of gas, how many city miles can
you drive without refi lling if you also make a 200-mile
highway trip?
19. A car starts out 400 miles from St. Louis and drives
directly toward St. Louis at 60 mph.
(Lesson 3-1)
a. Find an equation for the distance
d
from St. Louis
as a function of time
t
in hours from the start of the
trip.
b. Does the equation in Part a describe a
constant-increase or a constant-decrease situation?
20. The sum
S
of the measures of the interior angles of a
convex polygon varies directly as
n
-
2, where
n
is the
number of sides of the polygon.
(Lesson 2-1, Previous
Course)
a. Find the constant of variation.
b. Graph the function.
21. The independent variable of a function is given. State a
reasonable domain for the function.
(Lesson 1-4)
a.
h
=
number of hours worked in a day
b.
d
=
distance traveled away from home while on vacation
c.
t
=
temperature in Indianapolis, Indiana, in February
EXPLORATION
22.
Consider the lines
A
x +
B
y
=
C
and
B
x +
A
y
=
C
. Explore the
connections between slopes and intercepts of these lines.
QY ANSWERS
1. The slope is
–
A
__
B
, and
the y-intercept is
C
__
B
.
2. When C = 0, the x- and
y-intercepts are both zero.
The line passes through
the origin, and the x- and
y-intercepts are
not distinct.
Gateway Arch in St. Louis is
the tallest national monument
in the United States. The
shape of the arch is known
as a catenary curve.
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