Parallel And Perpendicular Lines Worksheet Page 22
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50Equations of Lines
•
Write an equation of a line given information
about its graph.
•
Solve problems by writing equations.
Cost of Cellular Service
Vocabulary
can the equation of a
can the equation of a
•
slope-intercept form
line describe the cost of
line describe the cost of
30
•
point-slope form
cellular telephone service?
cellular telephone service?
20
A certain cellular phone company
10
charges a flat rate of $19.95 per month
for service. All calls are charged $0.07
per minute of air time t. The total charge
10
20
30
40
50
C for a month can be represented by the
Minutes
equation C
0.07t
19.95.
WRITE EQUATIONS OF LINES
You may remember from algebra that an
equation of a line can be written given any of the following:
• the slope and the y-intercept,
• the slope and the coordinates of a point on the line, or
• the coordinates of two points on the line.
The graph of C
19.95 has a slope of 0.07, and it intersects the y-axis
0.07t
at 19.95. These two values can be used to write an equation of the line. The
slope-intercept form
of a linear equation is y
mx
b, where m is the slope of
the line and b is the y-intercept.
slope
y
mx
b
C
0.07t
19.95
y-intercept
Slope and y-Intercept
Example
Example
1
1
Write an equation in slope-intercept form of the line with slope of
4
and y-intercept of 1.
y
mx
b
Slope-intercept form
y
4x
1
m
4, b
1
The slope-intercept form of the equation of the line is y
4x
1.
Another method used to write an equation of a line is the point-slope form of a
point-slope form
linear equation. The
is y
y
m(x
x
), where (x
, y
) are the
1
1
1
1
coordinates of any point on the line and m is the slope of the line.
given point
(x
,
y
)
1
1
y
y
m(x
x
)
1
1
slope
Lesson 3-4 Equations of Lines 145
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