AP Calculus AB : Asymptotic behavior in terms of limits involving infinity

Study concepts, example questions & explanations for AP Calculus AB

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Example Questions

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Example Question #22 : Functions, Graphs, And Limits

Assume that a population of bunnies grows at a rate of  where  is the number of bunnies in the population at time . If the population begins with 14 bunnies, given unlimited time to grow, how many bunnies do you expect there to be in the population?

 

Note: No integration is required for this problem.

Possible Answers:

70

350

5

 The population will never stop growing.

700

Correct answer:

350

Explanation:

The equation given is a model of logistic growth. Note that one of the other common forms this equation might be given in is . What we want to know is what the population will be given unlimited time, or what  is.

Looking at the form of the derivative above, note that if we start with 14 bunnies, we get a positive derivative. This means that the population is increasing. As our function is continuous, the population will keep growing until the derivative hits 0. When does this occur?

In the above form, it should be more clear that the derivative is only 0 at  and . Thus, the population will keep growing, but never go above 350, because if it were to hit 350, the derivative would be 0 and growth would stop. (Alternatively, if the population were above 350, you can see the derivative would be negative and that the population would shrink back down to 350. Indeed, it wasn't necessary to tell you we started with 14 bunnies -- the limit will be the same for any positive starting value.)

Example Question #23 : Functions, Graphs, And Limits

The function  has a horizontal asymptote at y=0. What does the presence of the horizontal asymptote imply?

Possible Answers:

The function will produce only positive outputs.

At x=0 the function is undefined.

The limit of the function as x-values approach 0 tends to either positive or negative infinity.

None of the other answers.

The function is discontinuous at y=0.

Correct answer:

None of the other answers.

Explanation:

Unlike vertical asymptotes, horizontal asymptotes of certain functions may be crossed. In these cases, and in the case of the given function, the horizontal asymptote may be crossed and even have defined values laying on it (e.g., f(0)=0 for the function provided). 

 

What is meaningful about the horizontal asymptote in this example is that it suggests the behavior of the function at large magnitudes. The denominator will increase much faster than the numerator. That is to say that the  will grow exponentially larger the  in the numerator such that, at large values (both positive and negative), the function will output y-values tending toward y=0.

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