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The function f(x) = sin x is graphed.
The function f ( x ) = sin x oscillates between 1 and −1 as x ±
The function f(x) = tan x is graphed.
The function f ( x ) = tan x does not approach a limit and does not approach ± as x ±

Recall that for any base b > 0 , b 1 , the function y = b x is an exponential function with domain ( , ) and range ( 0 , ) . If b > 1 , y = b x is increasing over ` ( , ) . If 0 < b < 1 , y = b x is decreasing over ( , ) . For the natural exponential function f ( x ) = e x , e 2.718 > 1 . Therefore, f ( x ) = e x is increasing on ` ( , ) and the range is ` ( 0 , ) . The exponential function f ( x ) = e x approaches as x and approaches 0 as x as shown in [link] and [link] .

End behavior of the natural exponential function
x −5 −2 0 2 5
e x 0.00674 0.135 1 7.389 148.413
The function f(x) = ex is graphed.
The exponential function approaches zero as x and approaches as x .

Recall that the natural logarithm function f ( x ) = ln ( x ) is the inverse of the natural exponential function y = e x . Therefore, the domain of f ( x ) = ln ( x ) is ( 0 , ) and the range is ( , ) . The graph of f ( x ) = ln ( x ) is the reflection of the graph of y = e x about the line y = x . Therefore, ln ( x ) as x 0 + and ln ( x ) as x as shown in [link] and [link] .

End behavior of the natural logarithm function
x 0.01 0.1 1 10 100
ln ( x ) −4.605 −2.303 0 2.303 4.605
The function f(x) = ln(x) is graphed.
The natural logarithm function approaches as x .

Determining end behavior for a transcendental function

Find the limits as x and x for f ( x ) = ( 2 + 3 e x ) ( 7 5 e x ) and describe the end behavior of f .

To find the limit as x , divide the numerator and denominator by e x :

lim x f ( x ) = lim x 2 + 3 e x 7 5 e x = lim x ( 2 / e x ) + 3 ( 7 / e x ) 5 .

As shown in [link] , e x as x . Therefore,

lim x 2 e x = 0 = lim x 7 e x .

We conclude that lim x f ( x ) = 3 5 , and the graph of f approaches the horizontal asymptote y = 3 5 as x . To find the limit as x , use the fact that e x 0 as x to conclude that lim x f ( x ) = 2 7 , and therefore the graph of approaches the horizontal asymptote y = 2 7 as x .

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Find the limits as x and x for f ( x ) = ( 3 e x 4 ) ( 5 e x + 2 ) .

lim x f ( x ) = 3 5 , lim x f ( x ) = −2

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Guidelines for drawing the graph of a function

We now have enough analytical tools to draw graphs of a wide variety of algebraic and transcendental functions. Before showing how to graph specific functions, let’s look at a general strategy to use when graphing any function.

Problem-solving strategy: drawing the graph of a function

Given a function f , use the following steps to sketch a graph of f :

  1. Determine the domain of the function.
  2. Locate the x - and y -intercepts.
  3. Evaluate lim x f ( x ) and lim x f ( x ) to determine the end behavior. If either of these limits is a finite number L , then y = L is a horizontal asymptote. If either of these limits is or , determine whether f has an oblique asymptote. If f is a rational function such that f ( x ) = p ( x ) q ( x ) , where the degree of the numerator is greater than the degree of the denominator, then f can be written as
    f ( x ) = p ( x ) q ( x ) = g ( x ) + r ( x ) q ( x ) ,

    where the degree of r ( x ) is less than the degree of q ( x ) . The values of f ( x ) approach the values of g ( x ) as x ± . If g ( x ) is a linear function, it is known as an oblique asymptote .
  4. Determine whether f has any vertical asymptotes.
  5. Calculate f . Find all critical points and determine the intervals where f is increasing and where f is decreasing. Determine whether f has any local extrema.
  6. Calculate f . Determine the intervals where f is concave up and where f is concave down. Use this information to determine whether f has any inflection points. The second derivative can also be used as an alternate means to determine or verify that f has a local extremum at a critical point.

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Source:  OpenStax, Calculus volume 1. OpenStax CNX. Feb 05, 2016 Download for free at http://cnx.org/content/col11964/1.2
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