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Homework Help: Determine 1-1 function

  1. Mar 30, 2010 #1
    1. The problem statement, all variables and given/known data

    [1] f(x) = x^3-2
    [2] f(x) = x^4+2

    2. Relevant equations

    f(a) = f(b)
    a = b

    3. The attempt at a solution
    Assuming that f(a) = f(b), for which some a =/ b

    So for [1] f(x) = x^3-2, let assume a=/b, that f(a) = f(b)
    in the end I got, a^3 = b^3

    for [2], I did the same thing, and for f(x) = x^4+2, in the end I got a^4 = b^4

    Now I need to simply them down to a = b, so I think I need to take cubic root on both side, and square root on both side, respectively.

    Now why is [2] not a 1-1 function, while [1] is a 1-1 function?
    When I solve for x, for example, in the same of y = x^2, i get sqrt of y = x, so it is not a 1-1 function.

    What about x^3-2? I got cub root of y + 2 = x
    Last edited: Mar 30, 2010
  2. jcsd
  3. Mar 30, 2010 #2


    Staff: Mentor

    The first one is usually called the cube root. For the other you need a fourth root, not a square root.

    You don't need the cube root and fourth root, though. In fact, it's best to not use them. For the first problem you have
    a3 = b3 <==> a3 - b3 = 0.
    The last equation can be factored to give three solutions, two of which are complex.

    For the other problem, you have
    a4 = b4 <==> a4 - b4 = 0.
    This equation can be factored to give four solutions, two of which are complex.

    You can ignore the complex solutions. Can you find the real solutions to these equations?

  4. Mar 30, 2010 #3
    Hi, thanks.
    What I do not understand is how can I tell
    For your question, I didn't really think about those complex number (since I didn't even know there are complex numbers in those roots).
    I guess, from your information, you just literally pointed out that [1] there is only one a (b), and [2] there is two a, b that can still produce 0 (the value of y).
    Last edited: Mar 30, 2010
  5. Mar 31, 2010 #4


    Staff: Mentor

    Can you solve the equation a4 - b4 = 0 by factoring? This is the same as ((a2)2 - (b2)2 = 0.

    Can you solve the equation a3 - b3 = 0 by factoring?
  6. Mar 31, 2010 #5


    User Avatar
    Science Advisor

    [itex](-1)^3\no 1^3[/itex]

    [itex](-1)^4 = 1^5[/itex]
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