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4.4
The Natural Logarithm Function
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Section Outline

• The Natural Logarithm of x
• Properties of the Natural Logarithm
• Exponential Expressions
• Solving Exponential Equations
• Solving Logarithmic Equations
• Other Exponential and Logarithmic Functions
• Common Logarithms
• Maxs and Mins of Exponential Equations

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The Natural Logarithm of x
Definition Example
Natural logarithm of x Given the graph of y ex, the reflection of that graph about the line y x, denoted y ln x
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Properties of the Natural Logarithm
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Properties of the Natural Logarithm

1. The point (1, 0) is on the graph of y ln x
   because (0, 1) is on the graph of y ex.
2. ln x is defined only for positive values of x.
3. ln x is negative for x between 0 and 1.
4. ln x is positive for x greater than 1.
5. ln x is an increasing function and concave down.

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Exponential Expressions
EXAMPLE
Simplify.
SOLUTION
Using properties of the exponential function, we
have
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Solving Exponential Equations
EXAMPLE
Solve the equation for x.
SOLUTION
This is the given equation.
Remove the parentheses.
Combine the exponential expressions.
Add.
Take the logarithm of both sides.
Simplify.
Finish solving for x.
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Solving Logarithmic Equations
EXAMPLE
Solve the equation for x.
SOLUTION
This is the given equation.
Divide both sides by 5.
Rewrite in exponential form.
Divide both sides by 2.
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Other Exponential and Logarithmic Functions
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Common Logarithms
Definition Example
Common logarithm Logarithms to the base 10
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Maxs Mins of Exponential Equations
EXAMPLE
The graph of is
shown in the figure below. Find the coordinates
of the maximum and minimum points.
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Maxs Mins of Exponential Equations
CONTINUED
At the maximum and minimum points, the graph will
have a slope of zero. Therefore, we must
determine for what values of x the first
derivative is zero.
This is the given function.
Differentiate using the product rule.
Finish differentiating.
Factor.
Set the derivative equal to 0.
Set each factor equal to 0.
Simplify.
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Maxs Mins of Exponential Equations
CONTINUED
Therefore, the slope of the function is 0 when x
1 or x -1. By looking at the graph, we can
see that the relative maximum will occur when x
-1 and that the relative minimum will occur when
x 1.
Now we need only determine the corresponding
y-coordinates.
Therefore, the relative maximum is at (-1, 0.472)
and the relative minimum is at (1, -1).