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Linear Functions

Graphing Linear Functions in Standard Form

There are several different ways to graph a linear function. Sometimes, the way the rule of the function is written can dictate the simplest way to graph it. Below, the graphs of linear functions given in standard form will be explored.

Concept

Standard Form of a Line

A linear equation is in standard form if all terms for the x and y variables are on one side of the equation, and the constant is on the other side of the equation.

Ax+By=C

Here, A, B, and C are real numbers and A and B cannot both equal 0. Several combinations of A, B, and C can describe the same line, but representing them with the smallest possible integers is preferred.

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Exercise
Graph the linear function given by the equation using a table of values.
4x2y=7
Show Solution
Solution
To graph the function, we can create a table of values giving different points on the line. To do this, we'll substitute arbitrarily-chosen x-values into the equation to find the corresponding y-values. Let's start with x=0.
4x2y=7
402y=7
-2y=7
y=-3.5
One point on the line is (0,-3.5). We can use the same process for finding other points.
x 4x2y=7 y
1 412y=7 -1.5
2 422y=7 0.5
3 432y=7 2.5
4 442y=7 4.5

To draw the graph of the function, we can plot all five points in a coordinate plane and connect them with a line.

Theory

Graphing Linear Functions using Intercepts

A function's x- and y-intercepts are the points where the graph of a function intersects with the x- and y-axes, respectively. It's possible to use a linear function's intercepts to graph it.

Method

Finding the Intercepts of a Graph

The intercepts of a graph share an important feature. For all x-intercepts, the y-coordinate is 0, and for all y-intercepts, the x-coordinate is 0.
This can be used to find the intercepts of a graph when its rule is known. For example, consider the line given by the following equation.
2x+5y=10

Method

Finding the x-intercept

To find the x-intercept, y=0 can be substituted into the equation.
Next, solve the equation for x.
2x+50=10
2x=10
x=5
The x-intercept is (5,0).

Method

Finding the y-intercept

The y-intercept can be found in a similar way. Substitute x=0 into the equation and solve for y.
20+5x=10
5y=10
y=2
The y-intercept is (0,2).
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Exercise
The amusement park ride "Spinning Teacups" has two different sizes of cups, large and small. Large cups fit 6 people and small cups fit 4 people. Maximum capacity for each ride is 48 people. The equation
4x+6y=48
models this situation, where x is the number of small cups and y is the number of large cups. Graph the situation and interpret the intercepts.
Show Solution
Solution

Example

Finding the intercepts

To begin, we will find each of the intercepts. Starting with the x-intercept, we can substitute y=0 into the rule and solve for x.
4x+6y=48
4x+60=48
4x=48
x=12
The x-intercept is (12,0). To find the y-intercept we can substitute x=0 and solve for y.
4x+6y=48
40+6y=48
Solve for y
6y=48
y=8
The y-intercept is (0,8).

Example

Graphing the function

To graph the function, we can plot the intercepts in a coordinate plane, and connect them with a line.

Notice that the graph does not extend infinitely. This is because, since x and y represent the numbers of different cups, negative numbers should not be included.

Example

Interpreting the intercepts

We can interpret the intercepts in terms of what x and y represent. The x-intercept is (12,0). This means a ride with 12 small cups can not have any large cups, because the maximum capacity of people has already been met. Similarly, the y-intercept of (8,0), tells us that a ride with 8 large cups will not allow for any small cups.


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