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Math Help - Strange Square Root Problem

  1. #1
    Newbie
    Joined
    May 2007
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    Dalby, Australia
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    Arrow Strange Square Root Problem

    Ok, I need some kind of algebraic method to:

    - Determine which values of r give a real answer for t, and which values of r give a complex answer for t



    Now i have been solving this easy enough using the quadratic formula

    (let x= etc

    then x^2=r+(without the t=)

    x^2-x=r
    then x^2-x-r=0, solve using quadratic formula).

    Now my teacher said that my method of trialling different values works, but isn't quite what he is after, he wants some kind of algebraic method to show which values produce a complex or real answer, like just shove in a value and it spits out an answer.

    Thanks any help is appreciated

    By the way - This is rather urgent.

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  2. #2
    Newbie
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    Dalby, Australia
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    Somebody, anybody? Help please!!
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  3. #3
    Grand Panjandrum
    Joined
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    someplace
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    Quote Originally Posted by Scorpion View Post
    Ok, I need some kind of algebraic method to:

    - Determine which values of r give a real answer for t, and which values of r give a complex answer for t



    Now i have been solving this easy enough using the quadratic formula

    (let x= etc

    then x^2=r+(without the t=)

    x^2-x=r
    then x^2-x-r=0, solve using quadratic formula).

    Now my teacher said that my method of trialling different values works, but isn't quite what he is after, he wants some kind of algebraic method to show which values produce a complex or real answer, like just shove in a value and it spits out an answer.

    Thanks any help is appreciated

    By the way - This is rather urgent.
    I think this is the same as your method:

    t = sqrt(r+ sqrt(r + ...))

    so:

    t^2 = r + sqrt(r + sqrt(r+..)) = r + t

    so we have:

    t^2 - t + r = 0

    This has real roots for t if (and only if) the discriminant of this quadratic
    (that's the thing under the square root sign in the quadratic formula) is
    greater or equal to zero.

    In this case the discriminant is 1-4r, so we need:

    1-4r>=0

    or:

    1>=4r

    so we need:

    r<=1/4.

    RonL
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