Asymptotes: Videos & Practice Problems

Understanding rational functions involves identifying asymptotes, which are lines the graph approaches but never touches. Vertical asymptotes occur where the denominator equals zero, while horizontal asymptotes depend on the degrees of the numerator and denominator. For instance, if the degree of the numerator is less than that of the denominator, the horizontal asymptote is at $0$. Additionally, common factors can create holes in the graph, indicating removable discontinuities. Mastering these concepts is essential for graphing rational functions accurately.

Introduction to Asymptotes

When analyzing rational functions, understanding asymptotes is crucial for accurately graphing these functions. Asymptotes are lines that the graph approaches but never touches, providing insight into the function's behavior at extreme values. For instance, consider the rational function \( f(x) = \frac{1}{x} \). This function exhibits both horizontal and vertical asymptotes.

To identify the horizontal asymptote, we observe the behavior of the function as \( x \) approaches infinity or negative infinity. In this case, as \( x \) approaches either infinity, \( f(x) \) approaches 0, indicating a horizontal asymptote at \( y = 0 \). This can be expressed using limit notation: as \( x \to \infty \), \( f(x) \to 0 \) and as \( x \to -\infty \), \( f(x) \to 0 \).

Next, we examine the vertical asymptote, which occurs where the function is undefined. For \( f(x) = \frac{1}{x} \), the function is undefined at \( x = 0 \). As \( x \) approaches 0 from the right, \( f(x) \) increases without bound, approaching positive infinity. Conversely, as \( x \) approaches 0 from the left, \( f(x) \) decreases without bound, approaching negative infinity. This behavior can be denoted as follows: as \( x \to 0^+ \), \( f(x) \to \infty \) and as \( x \to 0^- \), \( f(x) \to -\infty \).

Rational functions can exhibit various configurations of asymptotes. Some may have no asymptotes, while others can have one or multiple vertical and horizontal asymptotes. The presence and location of these asymptotes depend on the specific function being analyzed. For example, a function may have a vertical asymptote at \( x = a \) and a horizontal asymptote at \( y = b \), which can be represented graphically with dashed lines to indicate that the graph approaches these lines but does not intersect them.

In summary, understanding asymptotes is essential for graphing rational functions. By identifying horizontal and vertical asymptotes, one can predict the end behavior of the function and accurately represent its graph. This foundational knowledge is vital for further exploration of rational functions and their properties.

Sketch the graph of the function $f\left(x\right)=\frac{1}{x^2}$. Identify the asymptotes on the graph.

• A. Vertical Asymptote: $x=0$, Horizontal Asymptote: None
• B. Vertical Asymptote: $x=0$, Horizontal Asymptote: $y=0$
• C. Vertical Asymptote: $x=0$, Horizontal Asymptote: $y=0$
• D. Vertical Asymptote: $x=1$ , Horizontal Asymptote: $y=0$

Answer: (C)

Determining Vertical Asymptotes

Understanding vertical asymptotes is crucial when analyzing rational functions. To determine the location of vertical asymptotes, we follow a straightforward process that mirrors finding the domain of the function. The key step is to set the denominator of the rational function equal to zero and solve for the variable.

Before proceeding, it's essential to express the function in its lowest terms. This involves factoring and canceling any common factors in the numerator and denominator. For example, consider the function:

\[f(x) = \frac{x + 2}{2(x - 3)}\]

First, we simplify this function. By canceling the common factor of \(x + 2\), we rewrite it as:

\[f(x) = \frac{1}{x - 3}\]

Next, we set the denominator equal to zero:

\[x - 3 = 0\]

Solving this gives us:

\[x = 3\]

This indicates that there is a vertical asymptote at \(x = 3\).

Let’s explore another example:

\[f(x) = \frac{2}{2x + 6}\]

Here, we first factor the denominator:

\[2x + 6 = 2(x + 3)\]

After canceling the common factor of 2, we have:

\[f(x) = \frac{1}{x + 3}\]

Setting the denominator to zero yields:

\[x + 3 = 0\]

Thus, we find:

\[x = -3\]

This means there is a vertical asymptote at \(x = -3\).

In a third example, consider:

\[f(x) = \frac{1}{x^2 - 9}\]

Since the numerator is already in its simplest form, we focus on the denominator. We set it equal to zero:

\[x^2 - 9 = 0\]

Solving this gives:

\[x^2 = 9\]

Taking the square root of both sides results in:

\[x = \pm 3\]

Therefore, there are two vertical asymptotes: one at \(x = 3\) and another at \(x = -3\). It is important to note that having multiple vertical asymptotes is a common occurrence in rational functions.

With these examples, you can practice finding vertical asymptotes for various rational functions by following the same steps: simplify the function, set the denominator to zero, and solve for \(x\).

Based only on the vertical asymptotes, which of the following graphs could be the graph of the given function? $f\left(x\right)=\frac{x^2-4x}{x^2-x-12}$

• A.
• B.
• C.

Answer: (B)

Determining Removable Discontinuities (Holes)

When analyzing rational functions, it's essential to identify both vertical asymptotes and holes in the graph. Vertical asymptotes occur where the denominator equals zero, while holes, also known as removable discontinuities, arise from common factors in the numerator and denominator that can be canceled out.

To find vertical asymptotes, you typically factor the function and set the denominator equal to zero. For example, if you have a function where the denominator factors to include a term like \(x - 3\), you would find a vertical asymptote at \(x = 3\).

However, to locate holes in the graph, you should identify any common factors in both the numerator and denominator without canceling them out. Instead, set these common factors equal to zero. For instance, if your function has a common factor of \(x + 2\), you would set \(x + 2 = 0\) and solve for \(x\), resulting in a hole at \(x = -2\). This hole is represented graphically with an open circle at that point on the curve.

Consider the function \(f(x) = \frac{x + 3}{x^2 + 4x + 3}\). Factoring the denominator gives \(f(x) = \frac{x + 3}{(x + 3)(x + 1)}\). Here, the common factor is \(x + 3\). Setting \(x + 3 = 0\) leads to a hole at \(x = -3\).

In another example, for the function \(f(x) = \frac{x^2 + 1}{x - 1}\), the numerator can be factored as \((x + 1)(x - 1)\). The common factor \(x - 1\) leads to a hole at \(x = 1\) when set to zero.

Understanding how to find holes in rational functions is crucial for accurately graphing these functions and interpreting their behavior. By recognizing both vertical asymptotes and holes, you can create a more complete picture of the function's graph.

Find all vertical asymptotes and holes of each function.
$f\left(x\right)=\frac{-5x}{\left(2x-3\right)^2}$

• A. Hole(s): $x=0$ , Vertical Asymptote(s): $x=\frac32$
• B. Hole(s): $x=\frac32$ , Vertical Asymptote(s): $x=\frac32$
• C. Hole(s): $x=0$ , Vertical Asymptote(s): $x=0$
• D. Hole(s): None , Vertical Asymptote(s): $x=\frac32$

Answer: (D)

Find all vertical asymptotes and holes of each function.
$f\left(x\right)=\frac{x^2-2x}{2x^3-x^2-6x}$

• A. Hole(s): None, Vertical Asymptote(s): $x=0,x=2,x=-\frac32$
• B. Hole(s): $x=0$, Vertical Asymptote(s): $x=-\frac32$
• C. Hole(s): $x=0$, $x=2$, Vertical Asymptote(s): $x=-\frac32$
• D. Hole(s): $x=0$, $x=2$, Vertical Asymptote(s): $x=\frac32$

Answer: (C)

Find all vertical asymptotes and holes of each function.
$f\left(x\right)=\frac{x^2+10x+25}{2x^2+8x-10}$

• A. Hole(s): None, Vertical Asymptote(s): $x=-5,$ $x=1$
• B. Hole(s): $x=-5$ , Vertical Asymptote(s): $x=1$
• C. Hole(s): $x=1$ , Vertical Asymptote(s): $x=-5$
• D. Hole(s): $x=-5$ , Vertical Asymptote(s): $x=-1$

Answer: (B)

Determining Horizontal Asymptotes

Understanding horizontal asymptotes is crucial for analyzing the behavior of rational functions, particularly how they influence the range. Unlike vertical asymptotes, which affect the domain, horizontal asymptotes are determined by the degrees of the numerator and denominator of a rational function.

To find horizontal asymptotes, we focus on two main scenarios based on the degrees of the numerator (the top part of the function) and the denominator (the bottom part). The degree of a polynomial is the highest power of the variable in that polynomial.

1. **Degree of Numerator Less Than Degree of Denominator**: If the degree of the numerator is less than the degree of the denominator, the horizontal asymptote is at y = 0. For example, in the function f(x) = 1/x, the degree of the numerator is 0 (no variable present) and the degree of the denominator is 1. Since 0 is less than 1, the horizontal asymptote is at y = 0.

2. **Degree of Numerator Equal to Degree of Denominator**: If the degrees are equal, the horizontal asymptote can be found by dividing the leading coefficients of the numerator and denominator. For instance, in the function f(x) = (2x + 3)/x, both the numerator and denominator have a degree of 1. The leading coefficient of the numerator is 2, and the leading coefficient of the denominator is 1. Thus, the horizontal asymptote is at y = 2/1 = 2.

3. **Degree of Numerator Greater Than Degree of Denominator**: If the degree of the numerator is greater than the degree of the denominator, there is no horizontal asymptote. Instead, the function may approach infinity or negative infinity as x approaches infinity or negative infinity.

To illustrate further, consider the function f(x) = 4x²/(-x³ - 5x + 9). The degree of the numerator is 2, and the degree of the denominator is 3. Since 2 is less than 3, the horizontal asymptote is at y = 0.

In another example, f(x) = (2x²)/(3x² + x - 1), both the numerator and denominator have a degree of 2. The leading coefficient of the numerator is 2, and that of the denominator is 3. Therefore, the horizontal asymptote is at y = 2/3.

It is also important to note that while horizontal asymptotes indicate the behavior of a function as x approaches infinity, the graph of the function may cross the horizontal asymptote. This phenomenon does not contradict the definition of asymptotes; it simply reflects the function's behavior in certain intervals.

In summary, determining horizontal asymptotes involves analyzing the degrees of the numerator and denominator, and applying the rules outlined above will help in understanding the behavior of rational functions effectively.

Find the horizontal asymptote of each function.
$f\left(x\right)=\frac{-5x}{\left(2x+3\right)^2}$

• A. Horizontal Asymptote at $y=0$
• B. Horizontal Asymptote at $y=-\frac54$
• C. Horizontal Asymptote at $y=-\frac52$

Answer: (A)

Find the horizontal asymptote of each function.
$f\left(x\right)=\frac{8x^2+1}{2x^2-x-6}$

• A. Horizontal Asymptote at $y=0$
• B. Horizontal Asymptote at $y=\frac14$
• C. Horizontal Asymptote at $y=4$

Answer: (C)

Find the horizontal asymptote of each function.
$f\left(x\right)=\frac{x^2+4x}{2x^3+8x^2}$

• A. Horizontal Asymptote at $y=0$
• B. Horizontal Asymptote at $y=\frac12$
• C. Horizontal Asymptote at $y=2$

Answer: (A)

Here’s what students ask on this topic:

What are asymptotes in rational functions?

Asymptotes in rational functions are lines that the graph of the function approaches but never touches. There are two main types of asymptotes: vertical and horizontal. Vertical asymptotes occur where the denominator of the rational function equals zero, indicating a value that the function cannot take. Horizontal asymptotes depend on the degrees of the numerator and denominator. If the degree of the numerator is less than the degree of the denominator, the horizontal asymptote is at y = 0. Understanding asymptotes is crucial for accurately graphing rational functions.

How do you find vertical asymptotes of a rational function?

To find vertical asymptotes of a rational function, follow these steps: First, write the function in its lowest terms by canceling any common factors. Then, set the denominator equal to zero and solve for x. The solutions to this equation are the x-values where the vertical asymptotes occur. For example, for the function f(x) = 1/(x-3), setting the denominator x-3 equal to zero gives x = 3, indicating a vertical asymptote at x = 3.

How do you determine horizontal asymptotes of a rational function?

To determine horizontal asymptotes of a rational function, compare the degrees of the numerator and the denominator. If the degree of the numerator is less than the degree of the denominator, the horizontal asymptote is at y = 0. If the degrees are equal, the horizontal asymptote is at y = (leading coefficient of the numerator)/(leading coefficient of the denominator). For example, for the function f(x) = 2x/(x+1), the degrees are equal, so the horizontal asymptote is at y = 2/1 = 2.

What is a hole in the graph of a rational function?

A hole in the graph of a rational function, also known as a removable discontinuity, occurs when a common factor in the numerator and denominator cancels out. To find the hole, set the common factor equal to zero and solve for x. This x-value is where the hole occurs. For example, for the function f(x) = (x+2)/(x+2)(x-3), the common factor x+2 cancels out, leaving a hole at x = -2.

Can a rational function have more than one vertical asymptote?

Yes, a rational function can have more than one vertical asymptote. This occurs when the denominator has multiple factors that can be set to zero. For example, the function f(x) = 1/((x-2)(x+3)) has vertical asymptotes at x = 2 and x = -3. Each factor in the denominator that equals zero represents a different vertical asymptote.