Evaluating limits of rational functions

In summary, the limit of (x^2+5)/(3x) as x approaches 0 goes to infinity because the numerator approaches a constant value while the denominator approaches 0. Without the +5, the function would approach 0 because the numerator and denominator have the same highest power. The comparison of highest powers method is not applicable in this case.
  • #1
artwill872
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Homework Statement


Why does the limit as x approaches 0 of
x^2 + 5 / 3x go to infinity (with 0 as an essential disc.) but without the +5, the function goes to 0?


Homework Equations





The Attempt at a Solution


I tried approaching evaluating the limit of the function by comparing the exponents of the numerator and denominator and that seems to work if the +5 isn't there. What is the explanation for this difference?
 
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  • #2
You mean (x^2+5)/(3x), right? Parentheses help. Without the +5, you've got x^2/(3x). That's simplifies to x/3, right? So it approaches 0. With the +5 the numerator approaches 5 and the denominator approaches 0, so the quotient is infinity. Counting powers doesn't really help here.
 
  • #3
A little more precisely, for x very, very close to 0, the numerator is close to 5 and the denominator is close to 0. That will be a very, very, large number. As x gets closer to 0, the numerator stays close to 5 while the denominator gets closer to 0. That is, the limit is [itex]+\infty[/itex] (which, since [itex]\infty[/itex] is not a real number is the same as saying the limit does not exist.

(Comparing highest powers is useful when x goes to [itex]\pm\infty[/itex].)
 

1. What is a rational function?

A rational function is a function that can be expressed as a ratio of two polynomials. It can also be written in the form f(x) = P(x) / Q(x), where P(x) and Q(x) are polynomials.

2. How do you evaluate the limit of a rational function?

To evaluate the limit of a rational function, you can use the factoring and cancellation method. First, factor both the numerator and denominator. Then, cancel out any common factors. Finally, plug in the limit value to see what the result is. If it is undefined, then the limit does not exist.

3. What are the steps to finding the horizontal asymptote of a rational function?

The horizontal asymptote of a rational function can be found by looking at the degree of the numerator and denominator. If the degree of the numerator is less than the degree of the denominator, the horizontal asymptote is y = 0. If the degrees are equal, divide the leading coefficients to find the horizontal asymptote. If the degree of the numerator is greater than the degree of the denominator, there is no horizontal asymptote.

4. Can a rational function have more than one vertical asymptote?

Yes, a rational function can have multiple vertical asymptotes. This occurs when there are zeros in the denominator of the function that cause the function to be undefined at those points.

5. How do you determine if a rational function is continuous or discontinuous?

A rational function is continuous at a certain point if the limit of the function exists at that point and is equal to the value of the function at that point. If the limit does not exist or is not equal to the function value at that point, then the function is discontinuous at that point.

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