How to Find Limits at Infinity

Step-by-step Guide to Find Limits at Infinity

Here is a step-by-step guide to finding limits at infinity:

Step 1: Understand the Concept of a Limit

Before diving into limits at infinity, you need to understand what a limit is. A limit describes the value that a function approaches as the input (or independent variable) approaches a certain number. For limits at infinity, that certain number is either positive or negative infinity.

Step 2: Recognize the Notation

• When we write \(lim_{x→∞}​f(x)=L\), we mean that as \(x\) becomes very large (approaches infinity), the function \(f(x)\) gets closer and closer to some value \(L\).
• Similarly, \(lim_{x→−∞​}f(x)=M\) means that as \(x\) becomes very negative (approaches negative infinity), \(f(x)\) approaches the value \(M\).

Step 3: Analyzing Polynomial Functions

Let’s start with polynomial functions such as \(f(x)=x^2\).

• As \(x\) increases to large positive numbers, \(x^2\) becomes very large. So, \(lim_{x→∞​}x^2=∞\).
• As \(x\) decreases to large negative numbers, \(x^2\) is still very large (since squaring a negative number gives a positive result). So, \(lim_{x→−∞}​x^2=∞\).

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Step 4: Rational Functions

For rational functions like \(f(x)=\frac{1}{x}​\), you have to consider the degree of the polynomial in the numerator and denominator.

• For \(f(x)=\frac{1}{x}​\)​, as \(x\) approaches infinity, \(\frac{1}{x}​\)​ gets smaller and smaller, approaching zero. So, \(lim_{x→∞}​\frac{1}{x}​=0\).
• The same logic applies when \(x\) approaches negative infinity. The function still approaches zero.

Step 5: Functions with Horizontal Asymptotes

A horizontal asymptote is a horizontal line that the graph of the function approaches as \(x\) goes to infinity or negative infinity.

• If \(lim_{x→∞}​f(x)=L\) and \(lim_{x→−∞​}f(x)=L\), then the line \(y=L\) is a horizontal asymptote to \(f(x)\).

Step 6: Dealing with More Complex Functions

For more complex functions, such as those with exponents or roots, you may need to use algebraic techniques to find the limit at infinity.

• Consider \(f(x)=\sqrt{x^2+x​}−x\). As �x goes to infinity, \(x^2\) grows much faster than \(x\), so you can consider \(x^2\) as the dominant term. By factoring out \(x^2\) inside the square root, you can simplify the expression to find the limit.

Step 7: Use L’Hôpital’s Rule

If you find an indeterminate form like \(\frac{∞}{∞}\)​ or \(\frac{0}{0}\)​, you may use L’Hôpital’s Rule.

• This rule states that if \(lim_{x→c​} \frac{f(x)}{g(x)}\)​ yields an indeterminate form, then under certain conditions, \(lim_{x→c​} \frac{f(x)}{g(x)}\)​\(=lim_{x→c​} \frac{f′(x)}{g′(x)}​\), where \(f′(x)\) and \(g′(x)\) are the derivatives of \(f(x)\) and \(g(x)\), respectively.

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Step 8: Infinite Limits and Infinite Asymptotes

Understand the difference between a horizontal asymptote and other kinds of asymptotes. For example:

• If \(lim_{x→a}​f(x)=∞\), this means that as \(x\) approaches \(a\), \(f(x)\) increases without bound. This is often the case with vertical asymptotes, where the function does not approach a finite value but instead goes off to infinity.

Step 9: Practice with Graphs

Use graphing to visualize limits at infinity.

• Look at the graph of a function as \(x\) increases or decreases without bound.
• Notice how the function behaves: does it level off (horizontal asymptote), go off to infinity (infinite limit), or oscillate without approaching any value?

Step 10: Abstract Functions and General Rules

Learn some general rules that often apply to limits at infinity.

• For instance, if a function \(f(x)\) is dominated by a term with the highest degree in \(x\), the behavior of that term often determines the limit at infinity.
• In the case of exponential functions, recognize that as \(x\) approaches infinity, \(e^x\) approaches infinity, and \(e^{−x}\) approaches zero.

Step 11: Review and Practice

Finally, practice is crucial.

• Work through multiple examples of different types of functions.
• Solve exercises from textbooks or online resources.
• Use limit calculators to check your work, but ensure you understand the steps.

Through this process, you’ll develop intuition for limits at infinity and be able to tackle a wide range of problems involving this concept.