Search Menu

Contents

Defining a Limit

page 1 of 2

Defining a Limit

Page 1

Page 2

Defining a Limit

Defining a Limit

Defining a Limit

A limit of a function is the value that function approaches as the independent variable of the function approaches a given value. The equation f (x) = t is equivalent to the statement "The limit of f as x goes to c is t." Another way to phrase this equation is "As x approaches c, the value of f gets arbitrarily close to t." This is the essential concept of a limit.

Here are some properties of limits.

x = a    

k = k, where k is a constant.    

xb = ab    

= if a≥ 0    

Here are some properties of operations with limits. Let f (x) = C, and g(x) = D.

kf (x) = kC, where k is a constant.    

(f (xg(x)) = C±D    

f (xg(x) = C×D    

= , if D≠ 0    

[f (x)]n = Cn    

The more formal definition of a limit is the following. f (x) = A if and only if for any positive number ε, there exists another positive number δ, such that if 0 < | x - a| < ε, then | f (x) - A| < δ. This definition basically states that if A is the limit of f as x approaches a, then any time f (x) is within ε units of a value A, another interval (x - δ, x + δ) exists such that all values of f (x) between (x - δ) and (x + δ) lie within the bounds (A - ε, A + ε). A simpler way of saying it is this: if you choose an x-value x1 which is very close to x = a, there always exists another x-value x0 closer to a such that f (x0) is closer to f (a) than f (x1).

A limit of a function can also be taken "from the left" and "from the right." These are called one-sided limits. The equation xâÜ’a-]f (x) = A reads "The limit of f (x) as x approaches a from the left is A." "From the left" means from values less than a -- left refers to the left side of the graph of f. The equation xâÜ’a+]f (x) = A means that the limit is found by calculating values of x that approach a which are greater than a, or to the right of a in the graph of f.

There are a few cases in which a limit of a function f at a given x-value a does not exist. They are as follows: 1) If xâÜ’a-]f (x)≠xâÜ’a+]f (x). 2) If f (x) increases of decreases without bound as x approaches a. 3) If f oscillates (switches back and forth) between fixed values as x approaches a. In these situations, the limit of f (x) at x = a does not exist.

Page 1

Page 2