8.8 Slope
Learning Objectives
- I can analyze vertical change and horizontal change so that you can calculate the slope of a line.
Introduction
Suppose you have a toy airplane, and upon takeoff, it rises 5 feet for every 6 feet that it travels along the horizontal. What would be the slope of its ascent? Would it be a positive value or a negative value? In this concept, you'll learn how to determine the slope of a line by analyzing vertical change and horizontal change so that you can handle problems such as this one.
Guided Learning
The pitch of a roof, the slant of a ladder against a wall, the incline of a road, and even your treadmill incline are all examples of slope.
The slope of a line measures its steepness (either negative or positive).
For example, if you have ever driven through a mountain range, you may have seen a sign stating, “10% incline.” The percent tells you how steep the incline is. You have probably seen this on a treadmill too. The incline on a treadmill measures how steep you are walking uphill. Below is a more formal definition of slope.
The slope of a line is the vertical change divided by the horizontal change.
In the figure below, a car is beginning to climb up a hill. The height of the hill is 3 meters and the length of the hill is 4 meters. Using the definition above, the slope of this hill can be written as . Because , we can say this hill has a 75% positive slope.
Similarly, if the car begins to descend down a hill, you can still determine the slope.
The slope in this instance is negative because the car is traveling downhill.
Another way to think of slope is: .
When graphing an equation, slope is a very powerful tool. It provides the directions on how to get from one ordered pair to another. To determine slope, it is helpful to draw a slope-triangle. A slope triangle is a triangle created from two points and whose side lengths are used to calculate the slope.
Using the following graph, choose two ordered pairs that have integer values such as (–3, 0) and (0, –2). Now draw in the slope triangle by connecting these two points as shown.
The vertical leg of the triangle represents the rise of the line and the horizontal leg of the triangle represents the run of the line. Another way to represent slope is:
Starting at the leftmost coordinate, count the number of vertical units and horizontal units it took to get to the rightmost coordinate.
Example A
Find the slope of the line graphed below.
Solution: Begin by finding two pairs of ordered pairs with integer values: (1, 1) and (0, –2).
Draw in the slope triangle.
Count the number of vertical units to get from the left ordered pair to the right.
Count the number of horizontal units to get from the left ordered pair to the right.
A more algebraic way to determine a slope is by using a formula. The formula for slope is:
The slope between any two points and is: .
represents one of the two ordered pairs and represents the other. The following example helps show this formula.
Take a minute to write all 3 formulas for finding slope in your notebook.
Example B
Using the slope formula, determine the slope of the equation graphed in Example A.
Solution: Use the integer ordered pairs used to form the slope triangle: (1, 1) and (0, –2). Since (1, 1) is written first, it can be called . That means
Use the formula:
As you can see, the slope is the same regardless of the method you use. If the ordered pairs are fractional or spaced very far apart, it is easier to use the formula than to draw a slope triangle.
Types of Slopes
Slopes come in four different types: negative, zero, positive, and undefined. The first graph of this Concept had a negative slope. The second graph had a positive slope. Slopes with zero slopes are lines without any steepness, and undefined slopes cannot be computed.
Any line with a slope of zero will be a horizontal line with equation .
Any line with an undefined slope will be a vertical line with equation .
We will use the next two graphs to illustrate the previous definitions.
Example C
To determine the slope of line , you need to find two ordered pairs with integer values.
(–4, 3) and (1, 3). Choose one ordered pair to represent and the other to represent .
Now apply the formula: .
To determine the slope of line , you need to find two ordered pairs on this line with integer values and apply the formula.
(5, 1) and (5, –6)
It is impossible to divide by zero, so the slope of line cannot be determined and is called undefined.
Guided Practice
Find the slope of each line in the graph below:
Check your work with a partner.
Solution:
For each line, identify two coordinate pairs on the line and use them to calculate the slope.
For the green line, one choice is and . This results in a slope of:
For the blue line, one choice is and . This results in a slope of:
The slopes can be seen in this graph:
Review
- Calculate slope by using one of the following methods:
Slope
The slope of a line is the vertical change divided by the horizontal change. The slope of a line measures its steepness (either negative or positive).
Slope-triangle
The slope-triangle is a right triangle whose hypotenuse is a portion of the graphed line, and where the two legs are vertical and horizontal segments whose lengths are used to calculate the slope.
Zero slope
A line with zero slope is a line without any steepness, or a horizontal line.
Undefined slope
An undefined slope cannot be computed. Vertical lines have undefined slopes.
Additional Resources
Slope and Rate of Change Video