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I don't understand what the range is about?

  1. Jun 10, 2007 #1


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    I don't understand what the range is about. Can someone please explain? Thanks in advance.
    Last edited: Jun 10, 2007
  2. jcsd
  3. Jun 10, 2007 #2
    The domain is the set values of the function in which the value would yeild a real number. for example: f(x)=5/x, x is not part of the domain as 5/0 is not real.

    The same, the range is the set of value from which if you put a number into the function these values can be found. For example: f(x)=x^2+1. The range is: f(x)=>1. This is because you cannot obtain numbers below 1 - put any number into the function and you will not be able to numbers below 0.

    to calculate the range, you simply find the inverse of the function and find the domain of the inverse function - or use differential calculus...
  4. Jun 10, 2007 #3
    But remember when finding the inverse of certain evil functions, that (SQRT(a))^2 or SQRT(a^2) = |a|, then set the other side of the equation to an inequality to RHS=>0. This trapped me a lot...
  5. Jun 10, 2007 #4

    matt grime

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    or you could just write it down. Note in the case given, your method of finding the domain of the inverse won't work, since the domain is constrained by x>10, meaning the range is y>0. Your method yields y=/=0.

    Just sketch the function, and look at it: it is always positive, and tends to infinty as x tends to 10 and 0 as x tends to infinity. It is continuous, hence the range is y>0.
    Last edited: Jun 10, 2007
  6. Jun 10, 2007 #5
    well 'simply' "write" this one "down" in one step:

  7. Jun 10, 2007 #6
    which "case" are you referring to?
  8. Jun 10, 2007 #7

    matt grime

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    The case in the OPs link that he was asking about. You've not written down the domains of either of your functions, so it is impossible to write the ranges. Assuming you mean 'for the maximal subset of R for which this expression yields a real number' then the first is clearly y=>0. What you need to do depends on the question. The OPs question is straight foward to do by inspection.
    Last edited: Jun 10, 2007
  9. Jun 10, 2007 #8
    matt grime: "Your method yields y=/=0"

    I am sorry, but you probably need to finish solving this inequality (which I assume you did) - so finally my method yields:


    Yes, I agree that my method does not couple the domain restriction of x>0. I have never encountered such a need to actually take into account a restiction before. This is one technique that I will find soon to implement upon this technique.
    Last edited by a moderator: Jun 10, 2007
  10. Jun 10, 2007 #9

    matt grime

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    Oh, and your method also presupposes that the function is injective - it need not and in general will not be invertible.
  11. Jun 10, 2007 #10
    If by injection, you mean one-one - it does not matter. Read post #3. I mean, give one example through which my method completly breaks down...

    Also (as I see it), the codomain is the set of all the Real values outputted by a function - thus this includes (unless stated otherwise), all the values of the function outputted by the domain.
    Last edited by a moderator: Jun 10, 2007
  12. Jun 12, 2007 #11
    Anyway Anived, remember post #3:
    For example: f(x)=(x^2-5x-9)^(1/4)
    To find the range, we can use my first method.

    therefore: y=(x^2-5x-9)^(1/4)
    therefore: y^4=|x^2-5x-9| -->
    now LHS cannot be less than 0 as RHS is an absolute value.
    thus we set:
    therefore: y=>0 ==>

    now back to our first result:
    now there are 2 possible equations that can be made from this. However, in functions, each number in the domain can only have 1 answer. So we take the positive half:
    therefore: y^4 = +(x^2-5x-9)
    therefore: y^4 = (x-5/2)^2 -9-25/4
    therefore: y^4 + 61/4 = (x-5/2)^2
    therefore: SQRT(y^4 + 61/4)=|x-5/2|
    now again set LHS => 0 (in this case it is unneccessary)
    SQRT(y^4 + 61/4)>0
    as you can see this yields yER==>
    now back to the original equation - noting that again the positive bit is taken once again:
    therefore: SQRT(y^4 + 61/4)=+(x-5/2)
    therefore: SQRT(y^4 + 61/4)+5/2 = x
    now the domain of this is also yER.==>
    Now I finally write this collecting all the results:
    Range = {y=>0}U{yER}U{yER}
    Range: {f(x)ER, f(x)=>0} ===>

    Note: I think I should learn Latex...
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