Rational Functions!
DEFINITION: a rational function is a polynomial divided by a polynomial
Examples:
AND
(corresponding to graphs seen below)
* Rational functions often have guidelines called
ASYMPTOTES (shown above in red)
- The first graph (see above) has a vertical asymptote of x=0 and a horizontal asymptote of y=0.
- The second graph has two VAs and one HA.
To FIND vertical asymptotes: set the denominator equal to zero.
Ex:
set x=0 ------ VA (x=0) ---- Domain is All Reals except 0
Ex:
set
to 0 ------ VA (x= +/- 2) ---- Domain: Reals except +/- 2
Ex:
set
to 0 ------ NO VA ---- Domain: All Reals
Algebraic Rules for horizontal asymptotes (HA) for the rational function:
- If n is less than m, then the HA is y=0 (x-axis)
If n=m, then the HA is y= a/b
If n>m, then there is no HA
Examples:
1.
------ 1<2 y="0
2.
------ 2=2 (n=m) ---- HA: y= 2/3
3.
------ 3>2 (n>m) ---- NO HA
REVIEW: Roots, Y-intercept, x/- Intervals, Symmetry
1. Roots: set numerator equal to zero ----
----
---- x= +/- 1
2. Y-intercept: sub in zero ----
=
=
------ (0, 1/4)
3. +/- Intervals: breakpoints (roots and discontinuities) {+/-1, +/-2}
VA: x= +/- 2
Then...
Test f(x) in each interval with a number line numbered from (in this case) -2 through 2
4. Symmetry: f(-x) = f(x) ---- Even (about y-axis)
5. VA: x= +/-2 HA: y=1
6. Sketch:
The EFFECT Of Multiplicity of Roots on Graph at VA's
is the same as
VA:
------ x=0, 4 (mult 2) *****
HA: 3=3 ---- 1/1 ---- y=1
Roots:
---- x= -2 and x=2 (mult 2)
Number line...
AND
The Actual Graph:
Common Factors in Numerator and Denominator ---- Hole in the Graph
Graph will be the same shape as
except undefined at x=2
On calculator table: for x=0 and 2 --- y will show "error"
** Cancel common factor -- shape of graph -- factors that cancel are the hole -- factors that don't cancel are the VA **
Algebraic Rules For Slant Asymptotes
A function f(x) has a slant asymptote iff ("if and only if") the degree of the numerator is exactly one more than the degree of the denominator.
and n-m=1
To sum things up:
- if n is less than m --- y=0
- n=m --- y=a/b
- n-m=1 --- slant
The equation of the slant asymptote is the quotient of the numerator divided by the denominator (disregard any remainder)
EXAMPLE:
You do long division just as you would with normal numbers.
- This comes out with a remainder of 27x-26
- Completely disregard the remainder
- The equation on top is the equation of the slant asymptote!
In the end, you get a slant asymptote of y=2x-7
Lastly...
GIVEN GRAPH, Write The Equation
1.
Roots: (1,0)
- Numerator: (x-1)
- VA: x=2
- Denominator: (x-2)
Solution:
2.
Roots: (-1,0) (1,0)
- Num: (x+1)(x-1)=
- VA: x=2, x=-2
- Denom: (x-2)(x+2)=
Solution:
3.
Roots: (0,0) mult 3
- Num:
- VA: x=3, x= -3
- Denom: (x-3)(x+3)=
Solution:
** Sometimes you just have to fool around with it in your calculator- plug in a negative or an integer.
** Just for the record, when a graph "bounces" at a root- the muliplicity of that root is 2.
And now for...
REVIEW PROBLEMS
1. Give the equation of the horizontal asymptote of
.
2. Give the equation of the slant asymptote of
.
3. Find the equations of all vertical asymptotes of
.
4. Write the equation of a rational function which has a horizontal asymptote of y=3/2 and y-intercept of -2.
5. Write the equation of a rational function which has roots 0 and -5 and whose vertical asymptotes are x=+/-2.
6. Given the graph, find the equation of the rational function.
7. Sketch the rational function
.
8. Find the correct equation for the graph below that has a SA of y= 1/2x + 5/4 and a VA of x= 1/2
9. Given the equation,
, find the horizontal and vertical asymptotes.
10. For the following equation,
, find the roots, the y-intercept, vertical asymptotes, horizontal asymptote, +/- intervals, symmetry, and domain.
---------------------------------------------------- Answers below --------------------
Answers:
1. HA: y= -4
2. SA: y= 2x+4
3. VA: x= -3/2 and x=2
4.
5.
6.
7.
8.
9.
VA: x= 5/2 and x= -1
HA: y= 5
10.
Roots:+/-i (no real roots)
Y-int: (0,1)
VA: x= +/- 1
HA: y=1
Intervals: - (less than -1), + (-1-0), + (0-1), - (greater than 1)
Symmetry: y-axis (x=0)
D: (-infinity, -1)(-1,1)(1, infinity)