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Section7.1 Characteristics of Logarithmic Functionssoln.notebook
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April 04, 2017
Unit 7 Logarithmic FunctionsSection 7.1 Characteristics of Logarithmic Function
with Base 10 and Base e
Section7.1 Characteristics of Logarithmic Functionssoln.notebook
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Inverse of a Function: We can determine an inverse by switching the x and y in the equation and then solving for y in
the new equation.
Note: When we switch the x and the y we essentially switch the domain and range. This results in the graphs being symmetrical with respect to the line y = x
Given the equation, y = x+2, determine its inverse. Graph both equations
Section7.1 Characteristics of Logarithmic Functionssoln.notebook
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Complete the table of values for the exponential function y = 10x, and its inverse x = 10y. Then graph both function and the reflection line y = x.
Original Function: y = 10x Inverse: x = 10y
State the domain and range of each function. What do you notice?
Section7.1 Characteristics of Logarithmic Functionssoln.notebook
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The inverse of an exponential function is known as a LOGARITHMIC FUNCTION, which can be written in either form.
has inverse:
OR
(exponential form)
(logarithmic form)
Note: > An exponential function has an x in the exponent while a
logarithmic function has a y in the exponent.> is read "log base b of x"
Section7.1 Characteristics of Logarithmic Functionssoln.notebook
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y = log10 x
This reads "y equals log base 10 of x", and it means: "what exponent would we have to raise 10 to in order to get x?” or “10 to what exponent will give you x?”
NOTE: The base of a logarithmic function can be some value other than 10, but 10 is the most common value. When we use the log button on a calculator, it automatically assumes a base of 10. Also, if we write a log equation without writing the base value in (ie. y = log x), then it is automatically assumed that the base is 10.
That is, log10 x means the same thing as log x.
In general, the inverse of the exponential function y = a(b)x can be written as x = a(b)y, or in logarithmic form as...
y = a log b x
where b > 0, b ≠1, a ≠ 0, and a and b are real numbers.
Most common type!
Section7.1 Characteristics of Logarithmic Functionssoln.notebook
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Complete the chart comparing exponential function and the logarithmic function
Use the log button on your calculator to try ad find log(3) and log(0). Can you explain the results?
Section7.1 Characteristics of Logarithmic Functionssoln.notebook
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Recall from last unit: For exponential functions of the form
• When b > 1, the graph is increasing
• When 0 < b < 1, the graph is decreasing
• a = initial value or the yintercept
Section7.1 Characteristics of Logarithmic Functionssoln.notebook
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Examining the Impact of the "a" value on a Logarithmic Function of the form y = a log10 x
When a > 0, the y values _________ as the xvalues _____________.
This is an increasing function from Quadrant ____ to Quadrant _____.
Look at the graphs of the following logarithmic functions:
1. y = log x 2. y = 3 log x 3. y = 5 log x
Section7.1 Characteristics of Logarithmic Functionssoln.notebook
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Look at the graphs of the following logarithmic functions:
4. y = log x 5. y = 4 log x 6. y = 6 log x
When a < 0, yvalues ____________ as the xvalues ______________
This is a decreasing function from Quadrant ____ to Quadrant _____
NOTE: Logarithmic functions do not have a yintercept but do have a restricted domain (i.e., x > 0) and hence a vertical asymptote at x = 0.
Section7.1 Characteristics of Logarithmic Functionssoln.notebook
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Natural Logarithms
Natural logarithms are special case of logs in which the base is "e", where e is a constant, irrational number... e = 2.718281828... (It stands for Euler.)
The constant "e" can be used as the base in an exponential function to give y = a(e)x. The inverse would be x = a(e)y. We can write this in logarithmic form as y = a loge x, or as y = a ln x (this format is the standard one for writing a logarithm with base "e"). A base "e" logarithm is called a natural logarithm.
Thus, when the base of an exponential is "e", the inverse will be the natural logarithm...y = a ln x.
Consider the graphs of y = ex and y = ln x
Section7.1 Characteristics of Logarithmic Functionssoln.notebook
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Comparing the Graphs of y = log10 x and y = ln x
Notice that changing the base of the logarithm from 10 to "e" slightly changes some of the yvalues. However, the properties of the graphs are the same.
Section7.1 Characteristics of Logarithmic Functionssoln.notebook
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Examples
Section7.1 Characteristics of Logarithmic Functionssoln.notebook
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Summary: Characteristics of the Graph of the Logarithm function
• The x intercept is 1.
• There is no y intercept.
• The yaxis is a vertical asymptote with equation x=0.
• Domain: {x|x > 0, x R}
• Range: {y|y R}
• is equivalent to , where x > 0 and b > 0, b =1
• b is the base of both the logarithmic exponential functions.
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