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Thread: More polynomial inequalities question

  1. #1
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    More polynomial inequalities question

    I had another question for the problem (x(x-8))/(x^2-9x-52)<=0

    my work is:
    x(x-8)/(x+4)(x-13)<=0

    i then did a number line with the numbers -4,0,8,13 and did the sign chart underneath. The answer I ended getting is (-4,0)U(8,13) I retried the problem several times and still got the same answer.

    I also did the problem (x-4)/(x+13)=>0 with that i got the answer (-infinity,-13]U[4,infinity)

    both answers were incorrect also.
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  2. #2
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    The answer for the first one you have is almost correct. It is $\displaystyle \left( { - 4,0} \right]\bigcup {\left[ {8,13} \right)}$ rather than $\displaystyle \left( { - 4,0} \right)\bigcup {\left( {8,13} \right)}$.

    Same deal with the second. It is $\displaystyle \left( { - \infty ,13} \right)\bigcup {\left[ {4,\infty } \right)}$ not $\displaystyle \left( { - \infty ,13} \right]\bigcup {\left[ {4,\infty } \right)}$.

    Those intervals can be tricky at times. If it is undefined at the endpoint, you should use the open interval. Also, always use an open interval for $\displaystyle \infty$, as you seem to know.

    Edit: forgot to add, the interval is closed in the first problem at 0 and 8 because those values make the function 0, not undefined. Since the inequality wants 0 included, the interval is closed. Hopefully that clarifies it a bit.
    Last edited by xifentoozlerix; Mar 28th 2008 at 02:00 PM.
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  3. #3
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    2. (-infinity,-13)U[4,infinity)
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  4. #4
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    The key idea to solving a problem like $\displaystyle \frac{x(x-8)}{(x+4)(x-13)} < 0$ is to use the fact that the numerator and denominator must have different signs. So either

    $\displaystyle x(x-8) > 0$ AND $\displaystyle (x+4)(x-13) < 0 $

    OR

    $\displaystyle x(x-8) < 0$ AND $\displaystyle (x+4)(x-13) > 0 $.

    Let's look at the first case:

    Question: When is $\displaystyle x(x+8) > 0$?
    Answer: When $\displaystyle x$ and $\displaystyle x+8$ are both positive OR both negative.

    $\displaystyle x > 0$ AND $\displaystyle x-8 > 0 \Rightarrow x > 8. \text{ } (8,\infty)$
    OR
    $\displaystyle x < 0$ and $\displaystyle x - 8 < 0 \Rightarrow x < 0. \text{ }(-\infty,0)$

    Conclusion 1:
    Putting this together we say the numerator is positive when $\displaystyle x$ is in $\displaystyle (-\infty,0) \cup (8,\infty)$

    Question: When is $\displaystyle (x+4)(x-13) < 0$?
    Answer: When $\displaystyle x+4$ and $\displaystyle x-13$ have opposite signs.

    This happens when:
    $\displaystyle x < -4$ AND $\displaystyle x > 13 $(this is not possible)
    OR
    $\displaystyle x > -4$ AND $\displaystyle x < 13$. $\displaystyle \text{ } (-4,13)$

    Conclusion 2:
    So we conclude that the denominator is negative when $\displaystyle x$ is in $\displaystyle (-4,13)$.

    Finally, we require that the numerator is positive AND the denominator is negative meaning both the first and second conclusions are met. This happens only when $\displaystyle x$ is in both intervals (their intersection) yielding as you pointed out $\displaystyle (-4,0) \cup (8,13)$. Now check when $\displaystyle \frac{x(x-8)}{(x+4)(x-13)} = 0$ and also check the other case: $\displaystyle x(x-8) < 0$ AND $\displaystyle (x+4)(x-13) > 0 $.

    Whenever you have an OR you are talking union:

    $\displaystyle x$ is in $\displaystyle A$ OR $\displaystyle B$ $\displaystyle \iff x $ is in $\displaystyle A \cup B. $

    Whenever you have an AND you are talking intersection:

    $\displaystyle x$ is in $\displaystyle A$ AND $\displaystyle B$ $\displaystyle \iff x $ is in $\displaystyle A \cap B. $
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