Today: Limits Involving Infinity

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Today: Limits Involving Infinity . Infinite limits. Limits at infinity. lim f(x) = L x -&gt; . lim f(x) =  x -&gt; a. Infinite Limits. CHAPTER 2. 2.4 Continuity. (see Sec 2.2, pp 98-101) . CHAPTER 2.

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Today: Limits Involving Infinity

Infinite limits

Limits at infinity

lim f(x) = L

x -> 

lim f(x) = 

x -> a

Infinite Limits

CHAPTER 2

2.4 Continuity

(see Sec 2.2, pp 98-101)

CHAPTER 2

DefinitionLet f be afunction definedon both sides of a, except possibly at a itself. Then

lim f(x) = 

x -> a

means that the values of f(x) can be made arbitrarily large by taking x close enough to a.

2.4 Continuity

Another notation for lim x -> a f(x) =  is

“f(x) --> as x --> a”

• For such a limit, we say:
• “the limit of f(x), as x approaches a, is infinity”
• “f(x) approaches infinity as x approaches a”
• “f(x) increases without bound as x approaches a”

Definition The line x = a is called a vertical asymptote of the curve y = f(x) if at least one of the following statements is true:

lim f(x) = lim f(x) = 

lim f(x) = -  lim f(x) = - .

x --> a -

x --> a+

x --> a +

x --> a -

Sec 2.6: Limits at Infinity

CHAPTER 2

2.4 Continuity

f(x) = (x2-1) / (x2 +1)

f(x) = ex

4

Sec 2.6: Limits at Infinity

CHAPTER 2

2.4 Continuity

f(x) = tan-1 x

f(x) = 1/x

Sec 2.6: Limits at Infinity

CHAPTER 2

2.4 Continuity

http://math.sfsu.edu/goetz/Teaching/math226f00/animations/limit.mov

animation

Definition: Limit at Infinity

Let f be a function defined on some interval (a, ). Then

lim f (x) = Lx ->

means that the values of f(x) can be made arbitrarily close to L by taking x sufficiently large.

Definition The line y = L is called a horizontal asymptote of the curve y = f(x) if either

lim f(x) = L or lim f(x) = L. x ->  x -> - 

lim tan-1(x)= - /2 x -> - 

lim tan –1(x) = /2. x -> 

If n is a positive integer, then

lim 1/ x n = 0 lim 1/ x n = 0. x-> -  x-> - 

lim e x = 0. x-> - 

We know lim x-> - e x = 0.

• What about lim x-> e x ?

f(x) = ex

Exponential Growth Model

• So lim t -> Ae rt =  for any r > 0.
• Say P(t) = Ae rt represents a population at time t.
• This is a mathematical model of “exponential growth,” where r is the growth rate and A is the initial population.
• See http://cauchy.math.colostate.edu/Applets

Exponential Growth/Decay

Forf(t) = Ae rt :

• Exponential growth (r > 0)
• Exponential decay (r < 0)

Logistic Growth Model

• A more complicated model of population growth is the logistic equation:
• P(t) = K / (1 + Ae –rt)
• What is lim t ->  P(t) ?
• In this model, K represents a “carrying capacity”: the maximum population that the environment is capable of sustaining.

Logistic Growth Model

• Logistic equation as a model of yeast growth

http://www-rohan.sdsu.edu/~jmahaffy/