6.06 Slope-intercept form

Here is a quick recap of what we know about straight lines on the coordinate plane:

• The slope is a measure of the steepness of a line
• An increasing line has a positive slope
• A decreasing line has a negative slope
• Slope can be determined using $\frac{\text{rise }}{\text{run }}$rise run ​
• A horizontal line has a slope of zero
• The slope of a vertical line is undefined
• The $x$x-intercept is the point where the line crosses the $x$x-axis
• The $y$y-intercept is the point where the line crosses the $y$y-axis

Slope-intercept form of a straight line

Any straight line on the coordinate plane is defined entirely by its slope and its $y$y-intercept. These two features are all we need to write an equation for the line.

The variable $a$a is used to represent the slope, and the variable $b$b is used for the $y$y-intercept.

We can represent the equation of any straight line, except vertical lines, using what is known as the slope-intercept form of a straight line.

To represent a particular line on the coordinate plane, with it's own unique slope and $y$y-intercept, we simply replace $a$a and $b$b with their corresponding values.

We can use the applet below to see the effect of varying $a$a and $b$b on both the line and its equation.

As can be seen from the applet, we can make the following statements about the slope.

Slope

The value of the slope, $a$a, relates to the line as follows:

• A negative slope ($a<0$a<0) means the line is decreasing
• A positive slope ($a>0$a>0) means the line is increasing
• A zero slope ($a=0$a=0) means the line is horizontal
• The higher the value of $a$a, the steeper the line.

Using the slope and $y$y-intercept to write the equation of a line

If we know the slope, $a$a, and $y$y-intercept, $b$b, of a line, we can substitute these values into $y=ax+b$y=ax+b to write the equation of the line.

Worked example

Example 1

Write the equation of a line that has a slope of $\frac{3}{4}$34​ and a $y$y-intercept of $-2$−2.

Solution

Substitute $a=\frac{3}{4}$a=34​ and $b=-2$b=−2 into $y=ax+b$y=ax+b

Practice questions

Question 1

Question 2

Use the slope and $y$y-intercept to graph a line

To graph a line from an equation, we use the slope, $a$a, and the $y$y-intercept, $b$b.

We begin by locating the $y$y-intercept as a point on the $y$y-axis.

From this point, we can use the slope to draw the correct slope of the line, as outlined in the three examples below:

Worked example

Example 2

Graph the line that has a slope of $4$4 and a $y$y-intercept of $-1$−1.

Solution

By comparing the equation of the line with $y=ax+b$y=ax+b, we see that the slope is $4$4 and the y-intercept is $-1$−1. The slope ($4=\frac{4}{1}$4=41​) tells us that for a 'run' of $1$1, we have a 'rise' of $4$4.

We can now create the graph on a coordinate plane, in a series of steps:

• Locate the $y$y-intercept at $-1$−1 on the $y$y-axis, and mark it with a point
• From the $y$y-intercept, we use the value for the slope to move $1$1 unit to the right and then $4$4 units up. This gives us the location of a second point.
• Draw a straight line between the two points, and extend the line beyond the points, across the entire coordinate plane.

Example 3

Graph the line with equation $y=-\frac{2}{3}x$y=−23​x.

Solution

By comparing the equation of the line with $y=ax+b$y=ax+b, we see that the slope is $-\frac{2}{3}$−23​ and the $y$y-intercept is $0$0, meaning the line passes through the origin. The slope tells us that for a 'run' of $3$3, we have a 'rise' of $-2$−2.

We can now create the graph on a coordinate plane, in a series of steps:

• Locate the $y$y-intercept at the origin, $\left(0,0\right)$(0,0), and mark it with a point
• From the $y$y-intercept, we use the value for the slope to move $3$3 units to the right and then $2$2 units down. This gives us the location of a second point.
• Draw a straight line between the two points, and extend the line beyond the points, across the entire coordinate plane.

Practice Question

Question 3

Equation of a horizontal line

A horizontal line has a slope of zero ($a=0$a=0), so the equation of the line becomes:

$y=b$y=b

where $b$b is the $y$y-intercept of the line.

Here are two examples of horizontal lines:

Horizontal lines $y=2$y=2 and $y=-3$y=−3

Equation of a vertical line

A vertical line has an undefined slope, so we can't use the slope-intercept form to write its equation. Instead we define a vertical line as having the equation:

$x=c$x=c

where $c$c is the $x$x-intercept of the line. 

Here are two examples of vertical lines:

Vertical lines $x=-1$x=−1 and $x=4$x=4

Practice question

Question 4