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Finding discontinuities - vertical asymptotes and holes - of rational functions

Vertical Asymptotes occur when factors in the denominator = 0 and do not cancel with factors in the numerator

  • Vertical asymptotes are vertical lines the graph approaches
  • The equation of the vertical asymptote is x = (that number which makes the denominator = 0)
Holes ( Removable Discontinuities ) occur when the factor in the denominator = 0 and it cancels with like factors in the numerator.
  • Holes are open "points" so they have an x and y coordinate
  • The x-value is the number that makes the cancelled factor = 0.
  • The y-value is found by substituting x into the "reduced" equation ( after cancelling) like factors.

Find the vertical asymptotes and holes (if any) for the following. Don't forget that vertical asymptotes are equations and holes are points!

y 1 x

Vertical Asymptote: x 0

Hole: None

y x x 1 x 1

Vertical Asymptote: None

Hole: (1,1) since (x-1) was cancelled, the hole is at x=1. To find the y-coordinate, plug 1 into the reduced equation: x x 1 x 1 x 1

y 4 x 3 x 7

Vertical Asymptote: x 7 since x 7 0

Hole: None

y 9 x 3 2 x

Vertical Asymptote: x 3 2 since 3 2 x 0 , x 3 2

Hole: None

y 7 x 9 x 1

Vertical Asymptote: x 9 , x 1 since x 9 and x 1

Hole: None

y 7 x 2 x 2 7 x 3

Vertical Asymptote: x 1 2 , x 3 since 2 x 2 7 x 3 0 , 2 x 1 x 3 0 , 2 x 1 0 and x 3 0 , x 1 2 and x 3

Hole: None

y 2 x 1 x 5 2 -1

Vertical Asymptote: x 5 since x 5 2 0 , x 5 0 , x 5

Hole: None

y x 3 x 2 25 -1

Vertical Asymptote: None since x 2 25 0 , x 2 25 , a number squared will never be negative

Hole: None

y x 7 x 2 2 -1

Vertical Asymptote: None since x 2 2 0 , x 2 2 and any number squared will never be a negative number

Hole: None

y 5 x 3

Vertical Asymptote: x 3 since x 3 0 , x 3 0 , x 3

Hole: None

y 4 x 4

Vertical asymptotes: x -4 and x 4 since x 4 0 , x 4 , x -4 and x 4

Hole: None

y 3 x 2 x 6 4 x 2 9

Vertical Asymptote: x -3

Hole: (3, 5 8 ) since 3 x 2 x 6 4 x 2 9 3 x 3 x 2 4 x 3 x 3 3 x 2 4 x 3 , (x-3) was cancelled, so the hole is at x=3. To find the y-coordinate, plug 3 into the reduced equation: 3 3 2 4 3 3 3 5 4 6 15 24 5 8

y -2 x 2 4 3 x 2 4 x 4

-2 x 2 4 3 x 2 4 x 4 -2 x 2 x 2 3 x 2 2 -2 x 2 3 x 2

Vertical Asymptote: x -2

Hole: None since the vertical asymptote takes care of the hole.

y x 2 4 x 2

Vertical Asymptote: None

Hole: (-2,-4) since x 2 4 x 2 x 2 x 2 x 2 x 2 , (x+2) was cancelled, so the hole is at x = -2. To find the y-coordinate, plug -2 into the reduced equation: -2 2 -4

y x 2 x 3 x 2 3 x

Vertical Asymptotes: None

Holes: (3,3), (0,0) since x 2 x 3 x 2 3 x x 2 x 3 x x 3 x , x and (x-3) were cancelled, so the holes are at x=0 and x=3. To find the y-coordinate, plug 0 and 3 into the reduced equation: 0, 3

y x 3 1 x 1

Vertical Asymptote: None

Hole: (1,3) since x 3 1 x 1 x 1 x 2 x 1 x 1 x 2 x 1 , (x-1) was cancelled, so the hole is at x=1. To find the y-coordinate, plug 1 into the reduced equation: 1 2 1 1 3

y 2 x 2 3 x 5 x 2 1

2 x 2 3 x 5 x 2 1 2 x 5 x 1 x 1 x 1 2 x 5 x 1

Vertical asymptote: x 1 since x 1 0

Hole: (-1, 7 2 ) Since (x+1) was cancelled, the hole is at x= -1. To find the y-coordinate, plug -1 into the reduced equation: 2 -1 5 -1 1 7 2

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Source:  OpenStax, "rational"ity. OpenStax CNX. May 03, 2006 Download for free at http://cnx.org/content/col10350/1.2
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