Rational Functions

A rational function is defined as the quotient of two polynomial functions.

Here are some examples of rational functions:

• g(x) = (x² + 1) / (x - 1)
• h(x) = (2x + 1) / (x + 3)

The rational functions to explored in this tutorial are of the form

Example: Find the domain of each function given below.

1. g(x) = (x - 1) / (x - 2)
2. h(x) = (x + 2) / x

Solution

1. For function g to be defined, the denominator x - 2 must be different from zero or x not equal to 2. Hence the domain of g is given by
   (-infinity , 2)U(2,+infinity).
   
   
2. For function h to be defined, the denominator x must be different from zero or x not equal to 0. Hence the domain of h is given by
   (-infinity , 0)U(0,+infinity).

Interactive Tutorial

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1. Click on the button "click here to start", above, to start the applet and maximize the window obtained.
   
2. Set a to 1, b to -1, c to 1 and d to -2 in order to define function g given in part a) of the above example. Check that the graph is discontinuous at x = 2 (no graph at x = 2).
   
3. Set a to 1, b to 2, c to 1 and d to 0 in order to define function h given in part b) of the above example. Check that the graph is discontinuous at x = 0 (no graph at x = 0).

What if the zeros of the numerator and the denominator of the rational function are equal?

Example
f(x) = (2x + 2) / (x + 1)
= 2(x + 1) / (x + 1)
= 2 , for x not equal to -1.

The graph of function f is a horizontal line with a hole (function not defined) at x = -1.

Interactive Tutorial

1. Go back to the applet window and set a to 2, b to 2, c to 1 and d to 1. Check that the graph is that of a horizontal line. It is not easy to observe the hole since the discontinuity (hole) in the graph has the dimension of one pixel which is very small to see.
   
2. Define another rational function with equal zeros in the numerator and denominator and check that the graph is that of a horizontal line.
   

Let f(x) = 1/x. f(x) is not defined at x = 0 (division by zero is not allowed). However what is the behavior of the graph "close" to zero?

In the tables below are values of function f as x approaches zero from the right (x >0) and as x approaches zero from the left (x < 0).

We note that as x approaches zero from the right, f(x) takes larger values. Is there a limit to the values of f(x)? No, f(x) increases without bound.

We also note that as x approaches zero from the left, f(x) takes smaller values. Is there a limit to the values of f(x)? No, f(x) decreases without bound. The vertical line x = 0 is called the vertical asymptote and it is given by the zero of the denominator.

Interactive Tutorial

1. Set parameters a to 0, b to 1, c to 1 and d to 0 (f(x) = 1/x). Observe the behaviour of the graph to the left and to the right of x = 0.
   
2. Set parameters a to 0, b to 1, c to 1 and d to different values (0, 1, -1,.... Observe the behaviour of the graph to the left and to the right of x = d.

Let f(x) = 1/x. What is the behavior of the graph of f as |x| becomes very large?

Tales below show values of f when x becomes very large, and when x becomes very small.

As x takes smaller values or as x takes larger values, f(x) takes values close to zero and the graph approaches the line horizontal line y = 0. This line is called the horizontal asymptote.

Interactive Tutorial

1. Set parameters a to 0, b to 1, c to 1 and d to 0 (f(x) = 1/x). Observe the behaviour of the graph as x takes large values (right) and as x takes smaller values (left). Note that the graph gets closer to the x axis (y = 0). Zoom in or out if necessary.
   
2. Set parameters a to 1, b to 1, c to 1 and d to 2. What is the equation of the horizontal asymptote? Change a and b only, set them to different non zero values and note that the equation of the horizontal asymptote is given by y = a/c.

Click on "click here to start" below to start the applet and generate graphs of rational functions of the form

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More on topics related to rational functions

tutorial on rational functions.

Graphs of rational functions

tutorial on graphs of rational functions

self test on graphs of rational functions.