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Y^2=f(x), y=f(x)

  1. Aug 7, 2007 #1
    hi guys, what is the relationship between y^2 = f(x) and y = f(x)?

    thank you.
     
  2. jcsd
  3. Aug 8, 2007 #2

    symbolipoint

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    Is that a trick question or is it a very highly theoretical and advanced question? You have indicated a situation in which y^2 = y. Best conclusion is y=1 and f(x)=1, horizontal line, one unit above the x axis.
     
  4. Aug 8, 2007 #3
    Or y=0...
    /*extra characters*/
     
  5. Aug 8, 2007 #4
    let's say we are given a function y=f(x), what is relationship between it and y^2 =f(x)?

    for example, if y=f(x) have a maximum point at (a,b), will y^2=f(x) have a maximum point at(a,b) too?

    i hope i make my question clear.:smile:
     
  6. Aug 8, 2007 #5
    it is not a trick question, nor a very highly theoretical question.
     
  7. Aug 8, 2007 #6
    Oh, you don't really mean y=f(x), and y2=f(x) do you? I think what you mean to say is if we're given a function f(x), then what is the relation between the function f(x), and the function (f(x))2.

    so lets let
    y=f(x) and
    z=(f(x))2

    Obviously z is always positive assuming we are only dealing with real numbers, but to investigate a relation about maxima/minima lets look at y'.

    y'=f'(x) and
    z'=2f(x)f'(x)

    if f(x) has a max/min at the point (a,b) then f'(a)=0
    Then y'(a)=0, and I think you can see then that z'(a)=2f(a)f'(a), but f'(a)=0 so z'(a)=0, thus the function z or (f(x))2 will also have a max or min at the point a, however this time it will be at the point (a, b2).

    I think it should be fairly easy to show that if f has a min at x=a then so does f2, and the same if f has a max at x=a, but I'm a bit too tired to try a proof of that at the moment.
     
  8. Aug 8, 2007 #7

    HallsofIvy

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    No, that's not how I would interpret the question.

    Let's clarify by taking f(x)= x. What is the relationship between y= x and y2= x?

    Suppose f(x)= x+ 3. What is the relationship between y= x+ 3 and y2= x+ 3?

    Frankly, I don't see much relationship. The first is a function and the second, for general f(x), is NOT a function. The first might be a square root of the second (if the first is positive for all x) but that was obvious wasn't it?
     
  9. Aug 8, 2007 #8

    Gib Z

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    I am quite sure this question is derived from a common one I've seen: Given a sketch of the graph of y=f(x), sketch y^2=f(x) labeling important features. d_leet's post works on that a bit. Also remember to find all points where y= 0 or 1, the graphs intersect there. Between zero and one, the y^2 graph will be slightly above the y graph. Other values, it will be below. You know the y^2 graph is discontinuous at the points where the y graph is negative.
     
  10. Aug 8, 2007 #9
    thank you everyone :smile:
    sorry that I didn't explain my question clearly. Anyway, Gib Z and HallsofIvy know what I mean :cool:

    but thanks d_leet too, I have learnt a way to prove from your post.
     
    Last edited: Aug 8, 2007
  11. Aug 8, 2007 #10
    That depends on whether f(a) is positive or negative. If f(a) is negative and a minimum, (f(a))^2 may be a maximum (e.g., if f(x) is the cosine function, then f(pi) is a minimum put (f(pi))^2 is a maximum).
     
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