Calculating Limits Using the Limit Laws
In this section we use the following properties of limits, called the Limit Laws, to calculate limits.
Calculating Limits Using the Limit Laws
These five laws can be stated verbally as follows:
Sum Law 1. The limit of a sum is the sum of the limits.
Difference Law 2. The limit of a difference is the difference of the limits.
Constant Multiple Law 3. The limit of a constant times a function is the constant times the limit of the function.
Calculating Limits Using the Limit Laws
Product Law 4. The limit of a product is the product of the limits.
Quotient Law 5. The limit of a quotient is the quotient of the limits (provided that the limit of the denominator is not 0).
For instance, if f(x) is close to L and g(x) is close to M, it is reasonable to conclude that f(x) + g(x) is close to L + M.
Example 1
Use the Limit Laws and the graphs of f and g in Figure 1 to evaluate the following limits, if they exist.
Figure 1
Example 1(a) – Solution
From the graphs of f and g we see that and
Therefore we have
(by Limit Law 1) (by Limit Law 3)
Example 1(b) – Solution
We see that limx →1 f(x) = 2. But limx →1 g(x) does not exist because the left and right limits are different:
So we can’t use Law 4 for the desired limit. But we can use Law 4 for the one-sided limits:
The left and right limits aren’t equal, so limx → 1 [f(x)g(x)]
does not exist.
cont’d
Example 1(c) – Solution
The graphs show that
and
Because the limit of the denominator is 0, we can’t use Law 5.
The given limit does not exist because the denominator approaches 0 while the numerator approaches a nonzero
Figure 1
cont’d
Calculating Limits Using the Limit Laws
If we use the Product Law repeatedly with g(x) = f(x), we obtain the following law.
In applying these six limit laws, we need to use two special limits:
These limits are obvious from an intuitive point of view (state them in words or draw graphs of y = c and y = x).
Power Law
Calculating Limits Using the Limit Laws
If we now put f(x) = x in Law 6 and use Law 8, we get another useful special limit.
A similar limit holds for roots as follows.
More generally, we have the following law.
Functions with the Direct Substitution Property are called continuous at a.
In general, we have the following useful fact.
Calculating Limits Using the Limit Laws
Calculating Limits Using the Limit Laws
Some limits are best calculated by first finding the left- and right-hand limits. The following theorem says that a
two-sided limit exists if and only if both of the one-sided limits exist and are equal.
When computing one-sided limits, we use the fact that the Limit Laws also hold for one-sided limits.
Calculating Limits Using the Limit Laws
The next two theorems give two additional properties of limits.
Calculating Limits Using the Limit Laws
The Squeeze Theorem, which is sometimes called the
Sandwich Theorem or the Pinching Theorem, is illustrated by Figure 7.
It says that if g(x) is squeezed between f(x) and h(x) near a, and if f and h have the same limit L at a, then g is forced to have the same limit L at a.
