1. 2.5 Limits Involving Infinity
Greg Kelly, Hanford High School, Richland, Washington

2. As the denominator gets larger, the value of the fraction gets smaller.
There is a horizontal asymptote if: or

3. This number becomes insignificant as .
Example 1:
This number becomes insignificant as
There is a horizontal asymptote at 1.

4. When we graph this function, the limit appears to be zero.
Find:
Example 2:
When we graph this function, the limit appears to be zero.
so for :
by the sandwich theorem:

5. Example 3:
Find:

6. Infinite Limits:
As the denominator approaches zero, the value of the fraction gets very large.
vertical asymptote at x=0.
If the denominator is positive then the fraction is positive.
If the denominator is negative then the fraction is negative.

7. The denominator is positive in both cases, so the limit is the same.
Example 4:
The denominator is positive in both cases, so the limit is the same.

8. End Behavior Models:
End behavior models model the behavior of a function as x approaches infinity or negative infinity.
A function g is:
a right end behavior model for f if and only if
a left end behavior model for f if and only if

9. becomes a right-end behavior model.
Example 7:
As , approaches zero.
(The x term dominates.)
becomes a right-end behavior model.
Test of
model
Our model
is correct.
As , increases faster than x decreases,
therefore is dominant.
becomes a left-end behavior model.
Test of
model
Our model
is correct.

10. becomes a right-end behavior model.
Example 7:
becomes a right-end behavior model.
becomes a left-end behavior model.
On your calculator, graph:
Use:

11. Right-end behavior models give us:
Example 7:
Right-end behavior models give us:
dominant terms in numerator and denominator

12. Often you can just “think through” limits.p