How do you graph #f(x)=(2x^2+1)/(x^3-x)# using holes, vertical and horizontal asymptotes, x and y intercepts?

1 Answer
Jan 18, 2018

We can graph this equation using a sign chart.

Explanation:

First, let's identify the holes, vertical and horizontal asymptotes and x and y intercepts.

Horizontal Asymptotes
Observing the formula, we can see the degree of the denominator is higher than the numerator (The degree is the largest exponent in a polynomial). This means there will be an asymptote at #y=0#.

Vertical Asymptotes
In order to find vertical asymptotes, we muct factor the denominator. Factoring out an #x# from #x^3-x#, we will get #x(x^2-1)#. Equating both answers to 0 will allow us to find our asymptotes.

#x=0#

#x^2-1=0#
#x^2=1#
#sqrt(x^2)=sqrt(1)#
#x=+-1#
#x!=-1,0,1#

#x# can not equal any of these values as they all result in a 0 in the denominator. This will make the function undefined aka asymptotes.

Holes
Holes occur when there is a zero in both the numerator and denominator that will cancel. There are none in this formula.

X-Intercepts
To find x-intercepts, substitute 0 for #f(x)# and solve.

#0=(2x^2+1)/(x^3-x)#

There are no x-intercepts as the equation above would result in imaginary numbers as answers (#(isqrt(2))/sqrt(2),-(isqrt(2))/sqrt(2)#).

Y-Intercepts
To find y-intercepts substitute 0 for #x# and solve.

#y=(2(0)^(2)+1)/((0)^(3)-(0))#

As the denominator equals zero the equation is undefined. Therefore there are no y-intercepts.

Sign Chart
Using the values above, also known as critical values, we will create a sign chart. We do this by sorting the critical values from least to greatest, and surrounding them with #-prop# and #prop#.

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We will then select a value for #x# in between the critical numbers.

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We will test the #x# values for positivity by substituting them for #x# in the formula and write either the positive or negative symbol in the spaces we created. I'll do #x=10# as an example below.
#y=(2(10)^(2)+1)/((10)^(3)-(10))#
#y=67/330#

We don't necessarily care about the value, just whether it is positive or negative. Since it is positive, we will mark a #+# between #1# and #prop#. We will do this for all the values we have chosen.

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We are now able to graph now that we know where the function is positive and negative and we have the asymptotes.
graph{(2x^2+1)/(x^3-x) [-10, 10, -5, 5]}
Notice how the graph is positive where we noted positive, and negative where we noted negative.