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Having trouble with complex quadratic equations

  1. Jan 2, 2007 #1
    All,

    I am not sure how to handle this situation with complex numbers. Can you help?

    Q = A^2 + ABi

    Where Q is a complex number.

    Can we not solve for A if we know Q and Bi using a quadratic equation?

    0 = A^2 + ABi + -Q

    Let...

    Q = 455
    Bi = -22

    And in this case I do know that A = 35. However...

    When,

    a = 1
    b = -22
    c = -455

    Solving...

    A = ( -1 +/- Sqrt { 1 - ( 4 * -22 * -455 )}) / 2

    I do not get A = 35. Any idea why and what I may be doing wrong?

    Thanks,

    Jeff
     
  2. jcsd
  3. Jan 2, 2007 #2

    Hurkyl

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    You messed up while substituting values into the quadratic formula.
     
  4. Jan 2, 2007 #3
    Show...

    Hurky,

    Could you show me what you mean? Where did I mess up?

    J
     
  5. Jan 2, 2007 #4
    You have confused what the problem is. You are using A in place of X, then you turn around and act like it is something else.
     
  6. Jan 3, 2007 #5

    Hurkyl

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    There are 5 places in the quadratic formula where you need to substitute a number. You got three of them wrong. You know the quadratic formula, right? The solutions to

    ax² + bx + c = 0

    are

    [tex]\frac{-b \pm \sqrt{b^2 - 4ac}}{2a},[/tex]

    right? Just double check your work!


    I'm quite sure he did not make that mistake.
     
  7. Jan 3, 2007 #6
    Oops...

    Hurky,

    Okay, I was placing a in place of b in the quadratic equation. Stupid mistake. Thanks greatly. Now I get the correct answer.

    Jeff
     
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