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Roots and absolute values

  1. Apr 22, 2014 #1
    1. The problem statement, all variables and given/known data

    I have a simple problem with roots and absolute values. When is the root of a number both negative and positive? Is only the equation of a number say f(x) = √x both the negative root and the positive root?

    2. Relevant equations

    If a = 1; b = -2, och x = a2√(ab-b2+2)

    Why is x only 2 and not -2 aswell?

    However if it were a function say f(x) the answer would be 2 and -2 right?

    Edit: For clarification how come √4 = 2 but f(4) = √4 = 2 and -2
    Last edited: Apr 22, 2014
  2. jcsd
  3. Apr 22, 2014 #2


    Staff: Mentor

    Given that we're talking about real roots of real numbers, an even root is not both negative and positive.
    The symbol √x represents the principal square root, which is a nonnegative number. For example, √4 = 2.
    I think you have a typo. If a = 1 and b = -2, then the quantity inside the radical is 1*(-2) - (-2)2 + 2 = -2 - 4 + 2 = -4.
    No. A function can have only one output value. Otherwise it's not a function.
  4. Apr 22, 2014 #3
    Yes there was a typo! x = a2√(ab+b2+2)

    When in a test do you know the difference between the principal square root and the root which gives you the answers +2 and -2? Because the answer from the test which the question was taken is 2. Why not both 2 and -2? Also I understand functions only have one y value but it can have to x values. It can't have two answers but it can have to inputs right?
    Last edited: Apr 22, 2014
  5. Apr 22, 2014 #4
    It is almost always the principal square root you should be concerned with.

    You might have confusion between √x2 and √4 .

    √4 = 2 but √x2 = |x|

    So if you have x2 = 4 ,you must have seen the solution as x= +2 ,-2 .

    This is because when you take square root on both the sides ,on the left you get |x| and on the right you get 2.

    x2 = 4
    √x2 = √4
    |x| = 2
  6. Apr 22, 2014 #5
    @Tanya Sharma thank you very much, your explanations are spot on!
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