Unit 3

Curve Sketching

Lesson 1: Symmetry and Asymptotes

Example 5

Determine the equation of the vertical asymptote on the graph of «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»§#8722;«/mo»«mn»4«/mn»«/mrow»«mrow»«msup»«mi»x«/mi»«mn»2«/mn»«/msup»«mo»§#8722;«/mo»«mn»4«/mn»«mi»x«/mi»«/mrow»«/mfrac»«/mrow»«/mstyle»«/math».

Solution

For «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»§#8722;«/mo»«mn»4«/mn»«/mrow»«mrow»«msup»«mi»x«/mi»«mn»2«/mn»«/msup»«mo»§#8722;«/mo»«mn»4«/mn»«mi»x«/mi»«/mrow»«/mfrac»«/mrow»«/mstyle»«/math», the denominator cannot equal zero.

«math style=¨font-family:Verdana¨ xmlns=¨http://www.w3.org/1998/Math/MathML¨»«mstyle mathsize=¨14px¨»«mtable columnalign=¨right center left¨»«mtr»«mtd»«msup»«mi»x«/mi»«mn»2«/mn»«/msup»«mo»§#8722;«/mo»«mn»4«/mn»«mi»x«/mi»«/mtd»«mtd»«mo»§#8800;«/mo»«/mtd»«mtd»«mn»0«/mn»«/mtd»«/mtr»«mtr»«mtd»«mi»x«/mi»«mi mathvariant=¨normal¨»(«/mi»«mi»x«/mi»«mo»§#8722;«/mo»«mn»4«/mn»«mi mathvariant=¨normal¨»)«/mi»«/mtd»«mtd»«mo»§#8800;«/mo»«/mtd»«mtd»«mn»0«/mn»«/mtd»«/mtr»«/mtable»«/mstyle»«/math»

Recall the graph of a function has a vertical asymptote at «math style=¨font-family:Verdana¨ xmlns=¨http://www.w3.org/1998/Math/MathML¨»«mstyle mathsize=¨14px¨»«mrow»«mi»x«/mi»«mo»=«/mo»«mi»a«/mi»«/mrow»«/mstyle»«/math» if the function can be written in the form «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»P«/mi»«mfenced»«mi»x«/mi»«/mfenced»«/mrow»«mrow»«mi»Q«/mi»«mfenced»«mi»x«/mi»«/mfenced»«/mrow»«/mfrac»«/mrow»«/mstyle»«/math», and «math style=¨font-family:Verdana¨ xmlns=¨http://www.w3.org/1998/Math/MathML¨»«mstyle mathsize=¨14px¨»«mrow»«mi»x«/mi»«mo»=«/mo»«mi»a«/mi»«/mrow»«/mstyle»«/math» makes the denominator, but not the numerator, equal to «math style=¨font-family:Verdana¨ xmlns=¨http://www.w3.org/1998/Math/MathML¨»«mstyle mathsize=¨14px¨»«mn»0«/mn»«/mstyle»«/math».

«math style=¨font-family:Verdana¨ xmlns=¨http://www.w3.org/1998/Math/MathML¨»«mstyle mathsize=¨14px¨»«mrow»«mi»x«/mi»«mo»§#8800;«/mo»«mn»0«/mn»«/mrow»«/mstyle»«/math» and «math style=¨font-family:Verdana¨ xmlns=¨http://www.w3.org/1998/Math/MathML¨»«mstyle mathsize=¨14px¨»«mrow»«mi»x«/mi»«mo»§#8800;«/mo»«mn»4«/mn»«/mrow»«/mstyle»«/math»

Simplify the function.

«math style=¨font-family:Verdana¨ xmlns=¨http://www.w3.org/1998/Math/MathML¨»«mstyle mathsize=¨14px¨»«mtable columnalign=¨right center left¨»«mtr»«mtd»«mi»y«/mi»«/mtd»«mtd»«mo»=«/mo»«/mtd»«mtd»«mfrac»«mrow»«mi»x«/mi»«mo»§#8722;«/mo»«mn»4«/mn»«/mrow»«mrow»«msup»«mi»x«/mi»«mn»2«/mn»«/msup»«mo»§#8722;«/mo»«mn»4«/mn»«mi»x«/mi»«/mrow»«/mfrac»«/mtd»«/mtr»«mtr»«mtd/»«mtd»«mo»=«/mo»«/mtd»«mtd»«mfrac»«mrow»«mi»x«/mi»«mo»§#8722;«/mo»«mn»4«/mn»«/mrow»«mrow»«mi»x«/mi»«mfenced»«mrow»«mi»x«/mi»«mo»§#8722;«/mo»«mn»4«/mn»«/mrow»«/mfenced»«/mrow»«/mfrac»«/mtd»«/mtr»«mtr»«mtd/»«mtd»«mo»=«/mo»«/mtd»«mtd»«mfrac»«mmultiscripts»«menclose notation=¨updiagonalstrike¨»«mi»x«/mi»«mo»-«/mo»«mn»4«/mn»«/menclose»«mprescripts/»«none/»«mn mathcolor=¨#FF0000¨»1«/mn»«/mmultiscripts»«menclose notation=¨updiagonalstrike¨»«msup»«mi»x«/mi»«mn mathcolor=¨#FF0000¨»1«/mn»«/msup»«mfenced»«mrow»«mi»x«/mi»«mo»-«/mo»«mn»4«/mn»«/mrow»«/mfenced»«/menclose»«/mfrac»«/mtd»«/mtr»«mtr»«mtd/»«mtd»«mo»=«/mo»«/mtd»«mtd»«mfrac»«mn»1«/mn»«mi»x«/mi»«/mfrac»«mi mathvariant=¨normal¨»,«/mi»«mo»§#160;«/mo»«mi»x«/mi»«mo»§#8800;«/mo»«mn»0«/mn»«mi mathvariant=¨normal¨»,«/mi»«mo»§#160;«/mo»«mn»4«/mn»«/mtd»«/mtr»«/mtable»«/mstyle»«/math»

The equation of the vertical asymptote is «math style=¨font-family:Verdana¨ xmlns=¨http://www.w3.org/1998/Math/MathML¨»«mstyle mathsize=¨14px¨»«mrow»«mi»x«/mi»«mo»=«/mo»«mn»0«/mn»«/mrow»«/mstyle»«/math».

Note that eliminating the common factor «math style=¨font-family:Verdana¨ xmlns=¨http://www.w3.org/1998/Math/MathML¨»«mstyle mathsize=¨14px¨»«mrow»«mi mathvariant=¨normal¨»(«/mi»«mi»x«/mi»«mo»§#8722;«/mo»«mn»4«/mn»«mi mathvariant=¨normal¨»)«/mi»«/mrow»«/mstyle»«/math» from the numerator and denominator does not change the restrictions on the domain. The variable «math style=¨font-family:Verdana¨ xmlns=¨http://www.w3.org/1998/Math/MathML¨»«mstyle mathsize=¨14px¨»«mi»x«/mi»«/mstyle»«/math» cannot be «math style=¨font-family:Verdana¨ xmlns=¨http://www.w3.org/1998/Math/MathML¨»«mstyle mathsize=¨14px¨»«mn»0«/mn»«/mstyle»«/math» or «math style=¨font-family:Verdana¨ xmlns=¨http://www.w3.org/1998/Math/MathML¨»«mstyle mathsize=¨14px¨»«mn»4«/mn»«/mstyle»«/math». However, since a common factor of «math style=¨font-family:Verdana¨ xmlns=¨http://www.w3.org/1998/Math/MathML¨»«mstyle mathsize=¨14px¨»«mrow»«mi mathvariant=¨normal¨»(«/mi»«mi»x«/mi»«mo»§#8722;«/mo»«mn»4«/mn»«mi mathvariant=¨normal¨»)«/mi»«/mrow»«/mstyle»«/math» can be eliminated from the numerator and denominator, «math style=¨font-family:Verdana¨ xmlns=¨http://www.w3.org/1998/Math/MathML¨»«mstyle mathsize=¨14px¨»«mrow»«mi»x«/mi»«mo»=«/mo»«mn»4«/mn»«/mrow»«/mstyle»«/math» is not a vertical asymptote. Instead, there is point of discontinuity on the graph of «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»§#8722;«/mo»«mn»4«/mn»«/mrow»«mrow»«msup»«mi»x«/mi»«mn»2«/mn»«/msup»«mo»§#8722;«/mo»«mn»4«/mn»«mi»x«/mi»«/mrow»«/mfrac»«/mrow»«/mstyle»«/math» at «math style=¨font-family:Verdana¨ xmlns=¨http://www.w3.org/1998/Math/MathML¨»«mstyle mathsize=¨14px¨»«mrow»«mi»x«/mi»«mo»=«/mo»«mn»4«/mn»«/mrow»«/mstyle»«/math».

The «math style=¨font-family:Verdana¨ xmlns=¨http://www.w3.org/1998/Math/MathML¨»«mstyle mathsize=¨14px¨»«mi»y«/mi»«/mstyle»«/math»-coordinate of the point of discontinuity can be determined by substituting the non-permissible «math style=¨font-family:Verdana¨ xmlns=¨http://www.w3.org/1998/Math/MathML¨»«mstyle mathsize=¨14px¨»«mi»x«/mi»«/mstyle»«/math»-value of «math style=¨font-family:Verdana¨ xmlns=¨http://www.w3.org/1998/Math/MathML¨»«mstyle mathsize=¨14px¨»«mrow»«mi»x«/mi»«mo»=«/mo»«mn»4«/mn»«/mrow»«/mstyle»«/math» into the simplified version of the function, «math style=¨font-family:Verdana¨ xmlns=¨http://www.w3.org/1998/Math/MathML¨»«mstyle mathsize=¨14px¨»«mrow»«mi»y«/mi»«mo»=«/mo»«mfrac»«mn»1«/mn»«mi»x«/mi»«/mfrac»«/mrow»«/mstyle»«/math».

Substituting «math style=¨font-family:Verdana¨ xmlns=¨http://www.w3.org/1998/Math/MathML¨»«mstyle mathsize=¨14px¨»«mrow»«mi»x«/mi»«mo»=«/mo»«mn»4«/mn»«/mrow»«/mstyle»«/math» into the equation of the unsimplified function would result in the indeterminate form, «math style=¨font-family:Verdana¨ xmlns=¨http://www.w3.org/1998/Math/MathML¨»«mstyle mathsize=¨14px¨»«mfrac»«mn»0«/mn»«mn»0«/mn»«/mfrac»«/mstyle»«/math», which was discussed in Unit 1B Lesson 1 Example 1 .

The point of discontinuity is at «math style=¨font-family:Verdana¨ xmlns=¨http://www.w3.org/1998/Math/MathML¨»«mstyle mathsize=¨14px¨»«mfenced»«mrow»«mn»4«/mn»«mi mathvariant=¨normal¨»,«/mi»«mo»§#160;«/mo»«mfrac»«mn»1«/mn»«mn»4«/mn»«/mfrac»«/mrow»«/mfenced»«/mstyle»«/math» and is shown on the graph of the function as an open circle.

The domain of the function is the set of all real numbers such that «math style=¨font-family:Verdana¨ xmlns=¨http://www.w3.org/1998/Math/MathML¨»«mstyle mathsize=¨14px¨»«mi»x«/mi»«/mstyle»«/math» does not equal «math style=¨font-family:Verdana¨ xmlns=¨http://www.w3.org/1998/Math/MathML¨»«mstyle mathsize=¨14px¨»«mn»0«/mn»«/mstyle»«/math» or «math style=¨font-family:Verdana¨ xmlns=¨http://www.w3.org/1998/Math/MathML¨»«mstyle mathsize=¨14px¨»«mn»4«/mn»«/mstyle»«/math».
«math style=¨font-family:Verdana¨ xmlns=¨http://www.w3.org/1998/Math/MathML¨»«mstyle mathsize=¨14px¨»«mrow»«mi»D«/mi»«mo»=«/mo»«mfenced open=¨{¨ close=¨}¨»«mrow»«mi»x«/mi»«mi mathvariant=¨normal¨»|«/mi»«mi»x«/mi»«mo»§#8800;«/mo»«mn»0«/mn»«mo»§#160;«/mo»«mi mathvariant=¨normal¨» «/mi»«mi»and«/mi»«mo»§#160;«/mo»«mi mathvariant=¨normal¨» «/mi»«mi»x«/mi»«mo»§#8800;«/mo»«mn»4«/mn»«mi mathvariant=¨normal¨»,«/mi»«mo»§#160;«/mo»«mi»x«/mi»«mo»§#8712;«/mo»«mi mathvariant=¨normal¨»R«/mi»«/mrow»«/mfenced»«/mrow»«/mstyle»«/math»

The graph of the function «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»§#8722;«/mo»«mn»4«/mn»«/mrow»«mrow»«msup»«mi»x«/mi»«mn»2«/mn»«/msup»«mo»§#8722;«/mo»«mn»4«/mn»«mi»x«/mi»«/mrow»«/mfrac»«/mrow»«/mstyle»«/math» of is shown below.

The amount of information that can be learned about a function’s graph from a simple investigation into its vertical asymptote(s) can be overwhelming at first. Although the goal of Examples 4 and 5 was to simply determine the equations of the vertical asymptotes, the equations on their own tell a very small piece of the graph’s story.

In Example 4, once the restriction on the variable was determined, limits were used to determine the behaviour of the function on either side of that restricted value (the vertical asymptote), which will prove helpful when it comes time to sketch curves.

In Example 5, some additional information was learned about the function’s graph – a point of discontinuity at «math style=¨font-family:Verdana¨ xmlns=¨http://www.w3.org/1998/Math/MathML¨»«mstyle mathsize=¨14px¨»«mfenced»«mrow»«mn»4«/mn»«mi mathvariant=¨normal¨»,«/mi»«mo»§#160;«/mo»«mfrac»«mn»1«/mn»«mn»4«/mn»«/mfrac»«/mrow»«/mfenced»«/mstyle»«/math». Because accuracy when sketching graphs is essential, consider looking at such additional information as gifts that will make the sketching process easier and more accurate. The good news is that with time and practice, you will become more efficient at identifying and using these additional bits of helpful information!