Algebra 1 - Semester 1 - Functions and Linear Equations
Sections: Introduction  |  Section 1  |  Section 2  |  Section 3  |  Section 4 |  Section 5

Section Four: Part 1 | Part 2 | Part 3 | Part 4 | Part 5 | Part 6 | Part 7 | Part 8

Algebra 1: Functions and Linear Equations : Section Four

Slope-Intercept Form

When Carmen started working in 2002, she kept track of her earnings each year. Here is a table of her earnings to 2005, rounded to the nearest thousand.

x (year)

y (annual earnings, $ thousands)









You can use this information to find the equation of a line in slope-intercept form. Before getting started, let’s review slope-intercept form.

 Slope-Intercept Form

  • y = mx + b
  • m is the slope
  • b is the y-intercept

Example 1:  Write an Equation Using a Slope and One Point
In the example above, one point on the line is (2003, 20,000) and the slope is 2,000.  Write an equation in slope-intercept form for the line that passes through the point with slope 2000.


Next, use this equation to predict future earnings for Carmen. When you use a linear equation to predict values beyond the data range, you are using linear extrapolation.

Example 2: Linear Extrapolation
Assuming that Carmen’s annual earnings will continue to increase at the same rate, find Carmen’s earnings in 2010, using the equation y = 2000x – 3,986,000.


Example 3:  Find an Equation Using a Point and Slope
If a line passes through the point (2, 6) with a slope of - ¾, what is its slope-intercept equation?


Example 4:  Find an Equation Using Two Points
Find the equation of the line which passes through (4, 5) and (-1, 3).


  To Write an Equation in Slope-Intercept Form

Using the Slope and One Point

Using Two Points

1. Substitute the values of m, x,
and y into y = mx + b.

1. Find the slope

2.  Solve for b.

2. Choose one of the points

3.  Substitute the values of m
and b into y = mx + b.

3.  Follow the steps for using the slope
and one point.

Example 5:  Population of California
According to the U.S. Census Bureau, these are the estimated population figures for California in the years shown in the table.

Use the estimates for 1990 and 2005 to write a linear equation that represents the data.


Example 6:  Using the Two Intercepts
Find an equation of the line with an x-intercept of -6 and a y-intercept of -2.


Practice Quick Practice

  1. Use linear extrapolation to predict the population of California in 2012 from the equation y = 0.42 x – 806.


  2. Write an equation of the line that passes through (6, -2) with slope -3.


  3. Write an equation of the line that passes through (-3, -1) with slope 2/3.


  4. Write an equation of the line that passes through (4, 1) and (6, -2).


  5. Write an equation of the line that passes through (-5, 2) and (6, 2).


  6. Write an equation of the line that has these intercepts:  x-intercept of 4 and y-intercept 3.


  7. These are the cumulative totals of Harry’s earnings in his job so far. He was paid for a training day and for the first four days of work last week. These are days five through eight.

    x (Day)





    y (Cumulative Pay)





    Use any two ordered pairs from the table to find the linear equation that represents Harry’s cumulative pay.



  1. What do the slope and y-intercept represent?


  2. Use linear extrapolation with the equation you found in number 7 above to predict Harry’s cumulative pay on day 10.


HWHW 7 (13 points)

Go to the left menu bar and click on the homework. You may do this more than one time to improve your score. Good luck!

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