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Homework Help: Find an equation of a rational function

  1. Dec 10, 2009 #1
    Find an equation of a rational function f(x)that satisfies the conditions. We're allowed to use a calculator, by the way. (:
    Okay, conditions:
    vertical asymptote: x=-5, x=0
    Horizontal asymptote: y=0
    x-intercept=7; f(1)=4




    On the test i had no idea how to do it, but after seeing her key,I somewhat understand. I looked in the textbook and was able to see where many things came from.
    a(x-7)/(x+5)(x)
    for one, i have no idea where the a and the x came from. I do, however, understand that the (x-7) is from the x-int. and that the (x+5) is from the v.a.
    4=a(1-7)/(1+5)(x) again, no idea where the a and x came from. I see that 4=f(1) which means that y=4 and x=1.
    4=a(-6)/(6)(1)
    I understand she plugged in that last x finally with a 1 and simplified everything else.
    4=-a, a=-4
    f(x)=-4(x-7)/x(x+5)
    I understand the final equation. I'd just love to know were she got that a and the x (by iself on the denominator) from! Thanks (:
     
  2. jcsd
  3. Dec 10, 2009 #2

    LCKurtz

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    The vertical asymptotes of x = -5 and 0 gave the factors of (x+5) and (x-0) in the denominator. The x intercept of 7 gives the (x-7) in the numerator.

    That's where the fraction [itex]\frac{x-7}{(x+5)(x)}[/itex] comes from. The second degree in the denominator vs. first degree in the numberator gives the horizontal asymptote of 0 for free. You have one condition left and only need to note that multiplying the fraction by a constant a doesn't change any of the above features and lets you get the last constraint.
     
  4. Dec 11, 2009 #3
    Ohhh okay thanks so much!!! :]
     
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