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Math Help - Which one of these functions do NOT have an inverse?

  1. #1
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    Angry Which one of these functions do NOT have an inverse?

    I know that it has to be the function that is 'many to one', but im having trouble finding that one.

    Heres the four answers to choose from.

    A) f(x) = 1 / (x - 1)

    B) f(x) = x^3 + 1

    C) f(x) = x^2 + 3 (where x is bigger/equal to 0)

    D) f(x) = x^3 - 2x

    I sort of worked out it it's going to be one to do with powers and negative numbers, which can be used to make the same product, but im can't seem to work out which one. I've only used trial and error really and a bit of logic. Can someone give me a hint or even better the answer and explanation?

    Thanks in advanced!

    ..and sorry about my lack of math script skills.
    Last edited by nugiboy; September 28th 2008 at 01:23 PM.
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  2. #2
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    find f(1) and f(-1) for answer (d)
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  3. #3
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    Quote Originally Posted by skeeter View Post
    find f(1) and f(-1) for answer (d)
    Sorry but i tried that and it doesn't work.

    f(1) = -1
    f(-1) = 1

    Im pretty sure anyway.
    Thanks though. Any other suggestions?
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  4. #4
    Super Member 11rdc11's Avatar
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    Did you try graphing them and using the horizontal line test?
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  5. #5
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    Quote Originally Posted by 11rdc11 View Post
    Did you try graphing them and using the horizontal line test?
    Hey thanks for the idea. I should have thought of that. I just tried all the functions through on autograph, and ive hit a small problem. Graph D won't draw. All the other graphs seem to pass the test, so by elimination im guessing its graph D which won't. But i don't want to make assumptions. Would graph d pass the test? I can't really tell if it will or not.
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  6. #6
    Super Member 11rdc11's Avatar
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    Nope D does not pass the test
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  7. #7
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    Quote Originally Posted by nugiboy View Post
    Sorry but i tried that and it doesn't work.

    f(1) = -1
    f(-1) = 1

    Im pretty sure anyway.
    Thanks though. Any other suggestions?
    sorry ... was thinking f(x) = x^3 - x rather than f(x) = x^3 - 2x

    fact that it has more than one root is reason enough.
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