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Homework Help: Help with Algebra

  1. Jul 23, 2010 #1
    1. The problem statement, all variables and given/known data

    I'm stuck on this problem please help me

    find the range of this equation

    y = (x^2 - 4)/(x^2 - x - 12)

    I know that it is [1/3,-infintiy]U[?,infinity]
    yes I'm having trouble with that ?
    At first I was like ok there should be a horizontal asymptote at y=1 there is in calculus deffinition the image as x approaches infinity from both sides right? or is just infinity... but ya it appraoches one from both sides but it corsses one at x=-8 check the graph so how do I find how low it goes at this section also I can't use a calculator well I can but am suppose to show all work with out using one i.e. have one to use not suppose to use it thanks

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Jul 23, 2010 #2
    Y = (x²-4)/(x²-x-12)

    If you factor out the top and bottom of the equation you end up with

    (x-2)(x+2) //Use the "difference of squares formula"


    With no further simplifying to do, the only course of action you can really take is to figure out what Y cannot be right? What would the denominator have to equal, for the equation to have no solution?
  4. Jul 23, 2010 #3
    -3 and 4... but that wasn't my question I was asking for the range... please help me with that
  5. Jul 23, 2010 #4
  6. Jul 23, 2010 #5
    well I read them still can't solve the problem :(
  7. Jul 23, 2010 #6
    apparently I do this I don't understand what this guy is saying... or were he got .337 I got 1/3...

    (-∞ , 0.337] U [ 0.969,∞)....look at the critical values and evaluate the function as well as doing the VA consideration .....and you certainly will need a numerical calculator to find the critical values and function values
  8. Jul 23, 2010 #7
    can someone please show me the calc to do this
  9. Jul 23, 2010 #8


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    QuarkCharmer gave you a hint. The numerator factors into (x+3)(x-4). So your function goes to infinity at x=(-3) and x=4. Use calculus to figure out what happens in the intervals (-infinity,-3), (-3,4) and (4,infinity) and put them together to get the total range.
  10. Jul 23, 2010 #9
    Can you please help me with this part... I have to use the chain rule right and the product rule to find the critical values correct then what do I do?
  11. Jul 23, 2010 #10


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    I think you just need the quotient rule to differentiate (x²-4)/(x²-x-12). Simplify it, set it equal to zero and solve for the critical points.
  12. Jul 23, 2010 #11
    what do I do after finding the critical points? Sorry i forgot how to do this stuff I should be able to find them eaisly but I don't remeber what to do after I find them...
  13. Jul 23, 2010 #12


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    It is kind of disappointing you don't know. The critical points are candidates for being maxes or mins on each interval. What you are basically doing is sketching a graph of y. Do that. Find the value at the critical points, find the limiting behavior at each side of x=-3 and x=4, and put that together with what you already know, that y=1 is a horizontal asymptote. Now draw a nice sketch of the function. Deduce the range from that.
  14. Jul 24, 2010 #13
    Yeah, super disappointing.

    After finding critical points, you could use a sign chart to determine which are maxima and which are minima
  15. Jul 24, 2010 #14
    ok well I posted this in the calc section sorry I posted it here didn't realize it would invovle calculus but ya I found the cirtical values still couldn't solve the problem though even with a sign chart... please help me
  16. Jul 24, 2010 #15


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    If you know the critical points you can sketch the function. Find out how it behaves around -3 and 4. Where is it positive and where is it negative?

  17. Jul 24, 2010 #16
    I found them it didn't do me anything check other topic
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