43 Warm Up

1
4-3 Warm Up
1. Name the constant of variation for the
equation of the line that passes through (2, -3)
and (8, -12).
2. Graph y -2x
Line will pass through (0, 0) and (1, -2)

• Write a direct variation equation. Assume
• that y varies directly as x. Then solve.
• If y 32 when x 8, find x when y 64.
• If y -24 when x -3, find y when x -2.

2
Math 8H
4-3 Graphing Equations in Slope-Intercept Form
Algebra 1 Glencoe McGraw-Hill
JoAnn Evans
3
When you know the slope and the y-intercept, you
can write the equation of a line.
The Slope-Intercept form of an equation is
slope
y-intercept
Slope is a numberthe variable x is not part of
the slope.
4
Write the equation of the line whose slope is 9
and whose y-intercept is 6.
Write the equation of the line whose slope is -3
and whose y-intercept is 5 .
Write the equation of the line whose slope is
and whose y-intercept is 4.
Write the equation of the line whose slope is -5
and whose y-intercept is 0.
5
Use a Graph to Write the Equation of a Line
The y-intercept (b) is -4. Two easily
identifiable points are (0, -4) and (2, 0). Walk
the slope between them.
y mx b
y 2x - 4
6
Use a Graph to Write the Equation of a Line
y mx b
y -3x 1
y 7x - 5
y x 4
7
Graph each equation
.
.
.
.
The y-intercept is -2.
The y-intercept is 4.
The slope says to go UP 3, OVER 4. Locate the
next point.
The slope says to go DOWN 1, OVER 3. Locate the
next point.
8
Graph each equation after changing to
Slope-Intercept form
.
.
.
.
9
In 1980, the population of California was
approximately 23,668,000. During the next 10
years, the population increased by approximately
617,100 people per year.
Source
U.S. Census Bureau
Let y CA population
Let x years since 1980
Population of California
1980 Population
Increase per year (rate of change)
Number of years since 1980



y 617,100 x 23,668,000
10
If the rate of change for a situation is given as
617,000 people per year, which do you know which
should be x and which should be y?
The word per is a clue. People per year means
people over years. In a slope, the change in y
is over the change in x.
11
y
The Population of California

35
30
25
Population (in millions)
20
15
10
5
x
1 2 3 4 5
6 7 8 9 10
Year 0 1980
What does the y-intercept represent?
1980 population
The slope represents the RATE OF CHANGE in the
population. Every year the population increased
(on average) by 617,100 people.
Use the equation to estimate what the population
was in 1990.
29,839,000
12
A car rental agency charges a flat fee of 25
plus 0.17 per mile to rent a compact car on
weekends. Write an equation to show the cost of
a car in terms of the number of miles driven.
Let c total cost of renting a car
Let x of miles the car is driven
Cost per mile
Number of miles driven
Total rental cost



Flat fee
c .17 x 25
13
c
50
cost of renting the car
45
40
35
30
25
x
0 10 20 30 40 50 60 70 80
90 100
of miles driven
What does the c-intercept represent?
Cost for 0 miles
42.00
What will be the cost after 100 miles?