There are three main forms of a linear function: slope-intercept form, standard form, and point-slope form. Each form is based on a different set of parameters and all forms can be used to identify and interpret the key features of a linear function.
In the applet, move the m slider and the b slider.
The slope, m, changes the steepness of the line.
The y-intercept, b, changes where the line crosses the y-axis.
This allows us to write the equation of a line in slope-intercept form:
The main advantage of slope-intercept form is that we can easily identify two key features: the slope and the y-intercept directly from the equation. This form is especially helpful when we want to graph a linear function.
With equations in slope-intercept form, we can efficiently identify two points that define the line and allow us to sketch the graph:
Consider the following equation:
2y=-x+12
Write the equation in slope-intercept form.
Graph the equation.
Is the point (8, 2) a solution to the equation?
A bathtub has a clogged drain, so it needs to be pumped out. It currently contains 30 gallons of water.
The table of values shows the linear relationship of the amount of water remaining in the tub, y, after x minutes.
\text{Time in minutes } (x) | 0 | 1 | 2 | 3 |
---|---|---|---|---|
\text{Water remaining in gallons } (y) | 30 | 28 | 26 | 24 |
Determine the linear equation in slope-intercept form that represents this situation.
Draw the graph of this linear relationship with a clearly labeled scale. Only show the viable solutions.
Describe how the graph would change if, instead, there were initially 40 gallons of water in the tub, and it emptied at 2.5 gallons per minute.
Imogen is a cyclist. She typically bikes at 15 \text{ mph}. She is doing a 50-mile bike ride for charity.
Draw a graph that shows her distance remaining throughout the 50-mile ride if she bikes her typical speed.
Write the linear equation that represents the graph in part (a).
Explain whether or not we can predict the distance remaining after 5 hours.
The slope-intercept form of a line is:
Slope-intercept form is useful when we know, or want to know the slope of the line and the y-intercept of the line.
The standard form of a linear relationship is a way of writing the equation with all of the variables on one side:
To draw the graph from standard form, we can find and plot the x- and y-intercepts or convert to slope-intercept form.
A special case of standard form is y=b, when m=0. This will be a horizontal line since the rise will be 0. Every y-value will be b for each value of x. The slope of any horizontal line will be zero.
We can also have vertical lines in the form x=a, where every value of x will be a for each value of y. If we calcualted the slope using any two points, we would get a value of 0 in the denominator. Therefore, we say slope of a vertical line is undefined.
Determine whether the equation \dfrac{1}{2}x+4y=16 is written in standard form.
A tour company travels to the Great Smoky Mountains National Park. They use a combination of buses and vans to get tourists to their destination. One bus can take 42 passengers, and one van can take 7 passengers. One day, they have 168 people register for the tour.
Write an equation in standard form that could be used to model the number of buses and vans they could use to transport all the people registered.
Graph the equation with an appropriate scale and labels.
Predict the number of vans that would be required if only 1 bus was available.
The standard form of a line is:
Standard form is useful when we know, or want to know both intercepts of the line.
When we are given the coordinates of a point on the line and the slope of that line, then the point-slope form can be used to state the equation of the line.
Coordinates can be given as an ordered pair in a table of values, read from a graph, or described in a scenario. The slope can be stated as a value, calculated from two points, read from a graph, or given as a rate of change in a scenario.
Point-slope form of a linear relationship:
We can also find the equation in point-slope form when given the coordinates of two points on the line by first finding the slope of the line.
We now have three forms of expressing the same equation, and each provides us with useful information about the line formed.
Drag the point on the graph to move the line and drag the slider to change the slope.
If we are given a point on the line and the slope of the line, we can find the equation of the line by substituting the values of the point and slope into the point-slope form of a linear equation. The slope-intercept form of the equation can be found by solving the point-slope form for y.
For each of the following equations, determine if they are in point-slope form, standard form, or slope-intercept form. If they are not in standard form, convert them to standard form.
y-3=-\dfrac{2}{5}\left(x+7\right)
y=4x-10
A line passes through the two points \left(-3,7\right) and \left(2,-3\right).
Write the equation of the line in point-slope form.
Determine whether the ordered pair (-10,21) lies on the same line as \left(-3,7\right) and \left(2,-3\right).
A carpenter charges for a day's work using the given equation, where y is the cost and x is the number of hours worked:y-125=50\left(x-2\right)
Draw the graph of the linear equation from the point-slope form. Clearly label the axes with labels, units, and an appropriate scale.
Predict the charge for 6 hours of work using the graph.
Give an example of a non-viable solution if the carpenter only uses this model for a maximum of 10 hours per day. Explain your answer.
The point-slope form of a line is:
Point-slope form is useful when we know or want to know the slope of the line and a point on the line.