Unit 6

Exponential and Logarithmic Functions

Lesson 3: Logarithmic Functions

Skill Builder

Determining the Equation of the Inverse

Mapping is a useful technique to determine the graph of the inverse of a function. However, it is not the only way to graph the inverse of a function. If the equation of the inverse can be determined, technology can be used to graph the inverse of the function.

When you were given a list of points on the graph of a function, or when you were given a graph from which to read points, you ‘switched’ the «math style=¨font-family:Verdana¨ xmlns=¨http://www.w3.org/1998/Math/MathML¨»«mstyle mathsize=¨14px¨»«mi»x«/mi»«/mstyle»«/math»- and «math style=¨font-family:Verdana¨ xmlns=¨http://www.w3.org/1998/Math/MathML¨»«mstyle mathsize=¨14px¨»«mi»y«/mi»«/mstyle»«/math»-coordinates to determine the corresponding points on the graph of the function’s inverse.

Given the equation of the function, the equation of the inverse can be determined by switching the «math style=¨font-family:Verdana¨ xmlns=¨http://www.w3.org/1998/Math/MathML¨»«mstyle mathsize=¨14px¨»«mi»x«/mi»«/mstyle»«/math» and «math style=¨font-family:Verdana¨ xmlns=¨http://www.w3.org/1998/Math/MathML¨»«mstyle mathsize=¨14px¨»«mi»y«/mi»«/mstyle»«/math» variables in the original equation, and then solving for «math style=¨font-family:Verdana¨ xmlns=¨http://www.w3.org/1998/Math/MathML¨»«mstyle mathsize=¨14px¨»«mi»y«/mi»«/mstyle»«/math».

a.
Algebraically determine the equation of the inverse of the function «math style=¨font-family:Verdana¨ xmlns=¨http://www.w3.org/1998/Math/MathML¨»«mstyle mathsize=¨14px¨»«mrow»«mi»f«/mi»«mfenced»«mi»x«/mi»«/mfenced»«mo»=«/mo»«mfrac»«mrow»«mi»x«/mi»«mo»+«/mo»«mn»8«/mn»«/mrow»«mn»3«/mn»«/mfrac»«/mrow»«/mstyle»«/math».

b.
Verify the equation graphically.

a.
Rewrite «math style=¨font-family:Verdana¨ xmlns=¨http://www.w3.org/1998/Math/MathML¨»«mstyle mathsize=¨14px¨»«mrow»«mi»f«/mi»«mfenced»«mi»x«/mi»«/mfenced»«mo»=«/mo»«mfrac»«mrow»«mi»x«/mi»«mo»+«/mo»«mn»8«/mn»«/mrow»«mn»3«/mn»«/mfrac»«/mrow»«/mstyle»«/math» as an equation in the form «math style=¨font-family:Verdana¨ xmlns=¨http://www.w3.org/1998/Math/MathML¨»«mstyle mathsize=¨14px¨»«mrow»«mi»y«/mi»«mo»=«/mo»«mi»f«/mi»«mfenced»«mi»x«/mi»«/mfenced»«/mrow»«/mstyle»«/math».

«math style=¨font-family:Verdana¨ xmlns=¨http://www.w3.org/1998/Math/MathML¨»«mstyle mathsize=¨14px¨»«mrow»«mi»y«/mi»«mo»=«/mo»«mfrac»«mrow»«mi»x«/mi»«mo»+«/mo»«mn»8«/mn»«/mrow»«mn»3«/mn»«/mfrac»«/mrow»«/mstyle»«/math»

Now, “switch” the «math style=¨font-family:Verdana¨ xmlns=¨http://www.w3.org/1998/Math/MathML¨»«mstyle mathsize=¨14px¨»«mi»x«/mi»«/mstyle»«/math» and «math style=¨font-family:Verdana¨ xmlns=¨http://www.w3.org/1998/Math/MathML¨»«mstyle mathsize=¨14px¨»«mi»y«/mi»«/mstyle»«/math» variables in the equation.

«math style=¨font-family:Verdana¨ xmlns=¨http://www.w3.org/1998/Math/MathML¨»«mstyle mathsize=¨14px¨»«mrow»«mi»x«/mi»«mo»=«/mo»«mfrac»«mrow»«mi»y«/mi»«mo»+«/mo»«mn»8«/mn»«/mrow»«mn»3«/mn»«/mfrac»«/mrow»«/mstyle»«/math»

Then, solve for «math style=¨font-family:Verdana¨ xmlns=¨http://www.w3.org/1998/Math/MathML¨»«mstyle mathsize=¨14px¨»«mi»y«/mi»«/mstyle»«/math».

«math style=¨font-family:Verdana¨ xmlns=¨http://www.w3.org/1998/Math/MathML¨»«mstyle mathsize=¨14px¨»«mtable columnalign=¨right center left¨»«mtr»«mtd»«mi»x«/mi»«/mtd»«mtd»«mo»=«/mo»«/mtd»«mtd»«mfrac»«mrow»«mi»y«/mi»«mo»+«/mo»«mn»8«/mn»«/mrow»«mn»3«/mn»«/mfrac»«/mtd»«/mtr»«mtr»«mtd»«mn»3«/mn»«mi»x«/mi»«/mtd»«mtd»«mo»=«/mo»«/mtd»«mtd»«mi»y«/mi»«mo»+«/mo»«mn»8«/mn»«/mtd»«/mtr»«mtr»«mtd»«mn»3«/mn»«mi»x«/mi»«mo»-«/mo»«mn»8«/mn»«/mtd»«mtd»«mo»=«/mo»«/mtd»«mtd»«mi»y«/mi»«/mtd»«/mtr»«/mtable»«/mstyle»«/math»

The equation of the inverse of the function is «math style=¨font-family:Verdana¨ xmlns=¨http://www.w3.org/1998/Math/MathML¨»«mstyle mathsize=¨14px¨»«mrow»«mi»y«/mi»«mo»=«/mo»«mn»3«/mn»«mi»x«/mi»«mo»-«/mo»«mn»8«/mn»«/mrow»«/mstyle»«/math» or «math style=¨font-family:Verdana¨ xmlns=¨http://www.w3.org/1998/Math/MathML¨»«mstyle mathsize=¨14px¨»«mrow»«msup»«mi»f«/mi»«mrow»«mo»-«/mo»«mn»1«/mn»«/mrow»«/msup»«mfenced»«mi»x«/mi»«/mfenced»«mo»=«/mo»«mn»3«/mn»«mi»x«/mi»«mo»-«/mo»«mn»8«/mn»«/mrow»«/mstyle»«/math».

b.
The graph of both the function and its inverse should be reflections in the line «math style=¨font-family:Verdana¨ xmlns=¨http://www.w3.org/1998/Math/MathML¨»«mstyle mathsize=¨14px¨»«mrow»«mi»y«/mi»«mo»=«/mo»«mi»x«/mi»«/mrow»«/mstyle»«/math». Any point on the graph of the inverse should also correspond to a point on the graph of the original function with its «math style=¨font-family:Verdana¨ xmlns=¨http://www.w3.org/1998/Math/MathML¨»«mstyle mathsize=¨14px¨»«mi»x«/mi»«/mstyle»«/math»- and «math style=¨font-family:Verdana¨ xmlns=¨http://www.w3.org/1998/Math/MathML¨»«mstyle mathsize=¨14px¨»«mi»y«/mi»«/mstyle»«/math»-coordinates switched. For example, the point «math style=¨font-family:Verdana¨ xmlns=¨http://www.w3.org/1998/Math/MathML¨»«mstyle mathsize=¨14px¨»«mfenced»«mrow»«mo»-«/mo»«mn»5«/mn»«mo»,«/mo»«mo»§#160;«/mo»«mn»1«/mn»«/mrow»«/mfenced»«/mstyle»«/math» on the function of «math style=¨font-family:Verdana¨ xmlns=¨http://www.w3.org/1998/Math/MathML¨»«mstyle mathsize=¨14px¨»«mrow»«mi»f«/mi»«mfenced»«mi»x«/mi»«/mfenced»«mo»=«/mo»«mfrac»«mrow»«mi»x«/mi»«mo»+«/mo»«mn»8«/mn»«/mrow»«mn»3«/mn»«/mfrac»«/mrow»«/mstyle»«/math» becomes the point «math style=¨font-family:Verdana¨ xmlns=¨http://www.w3.org/1998/Math/MathML¨»«mstyle mathsize=¨14px¨»«mfenced»«mrow»«mn»1«/mn»«mo»,«/mo»«mo»§#160;«/mo»«mo»-«/mo»«mn»5«/mn»«/mrow»«/mfenced»«/mstyle»«/math» on the inverse function, «math style=¨font-family:Verdana¨ xmlns=¨http://www.w3.org/1998/Math/MathML¨»«mstyle mathsize=¨14px¨»«mrow»«msup»«mi»f«/mi»«mrow»«mo»-«/mo»«mn»1«/mn»«/mrow»«/msup»«mfenced»«mi»x«/mi»«/mfenced»«mo»=«/mo»«mn»3«/mn»«mi»x«/mi»«mo»-«/mo»«mn»8«/mn»«/mrow»«/mstyle»«/math».

The graphs are shown below.



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