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Math Help - Cubic Function behaviour

  1. #1
    Super Member sakonpure6's Avatar
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    Cubic Function behaviour

    Cubic Function behaviour-20140103_192427.jpg

    I say the answer is d) . But the book says its b) ?
    Who is wright?
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    Re: Cubic Function behaviour

    would it be so hard to rotate the picture before you post it? Really....
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  3. #3
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    Re: Cubic Function behaviour

    It wouldn't be so terribly difficult to type the whole thing so people do not have to try to read photographs!

    Which statement is true for any cubic polynomial function?
    (a) As x goes to +/- infinity, y goes to infinity.
    (b) As x goes to +/- infinity, the signs are opposite.
    (c) As x goes to +/- infinity, y goes to -infinity.
    (d) As x goes to +/- infinity, the signs are the same.
    Did you give this much thought at all? A simple cubic function is f(x)= x^3. "10000" isn't "infinity" but is pretty large! What is f(10000)? What is f(-10000)?
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    Super Member sakonpure6's Avatar
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    Re: Cubic Function behaviour

    f(-10000) = -10000^3 , the answer is negative. So it is option d) ???
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    Re: Cubic Function behaviour

    why don't you go through each of those choices and say whether or not it's true and if not why not. That will let you truly understand that problem.
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    Super Member sakonpure6's Avatar
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    Re: Cubic Function behaviour

    When it says the signes are the same what does that mean?
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    Re: Cubic Function behaviour

    Quote Originally Posted by sakonpure6 View Post
    When it says the signes are the same what does that mean?
    that's a good question. It's terrible terminology. What I assume it means is that

    (x>0) \longrightarrow (x^3>0) $ and $(x<0) \longrightarrow (x^3 < 0)
    Last edited by romsek; January 4th 2014 at 06:23 PM.
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  8. #8
    Super Member sakonpure6's Avatar
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    Re: Cubic Function behaviour

    I am really convinced its option d) because x -> infinity then y -> infinity and when x -> -infinity then so will y.
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    Re: Cubic Function behaviour

    Quote Originally Posted by sakonpure6 View Post
    I am really convinced its option d) because x -> infinity then y -> infinity and when x -> -infinity then so will y.
    well that's good because (d) is indeed correct.
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    Super Member sakonpure6's Avatar
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    Re: Cubic Function behaviour

    thank you all!
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    Re: Cubic Function behaviour

    Quote Originally Posted by romsek View Post
    well that's good because (d) is indeed correct.
    What? (d) is
    (d) As x goes to +/- infinity, the signs are the same.
    but sakonpure6 has already said that f(-10000) is negative while f(10000) is positive.
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  12. #12
    Super Member sakonpure6's Avatar
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    Re: Cubic Function behaviour

    @Halls I am really confused, can you please explain your self?
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    Re: Cubic Function behaviour

    You said earlier you did not know what "the signs are the same" meant. f(1000) is positive. Its "sign" is "+". f(-1000) is negative. Its sign is "-". The signs are NOT "the same".
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  14. #14
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    Re: Cubic Function behaviour

    I think the trouble is in the terrible wording of the multiple choices.

    If one assumes that "the signs are the same" means what I said a few posts back then (d) is correct. If not, well you'll have to define what it means then.

    so you don't have to scroll back..

    (x>0) \longrightarrow (x^3>0) $ and $(x<0) \longrightarrow (x^3 < 0)
    Last edited by romsek; January 5th 2014 at 06:44 PM.
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