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Having trouble with complex quadratic equations

  1. Jan 2, 2007 #1

    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


    Q = 455
    Bi = -22

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


    a = 1
    b = -22
    c = -455


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

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


  2. jcsd
  3. Jan 2, 2007 #2


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    Gold Member

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


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

  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


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    Staff Emeritus
    Science Advisor
    Gold Member

    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


    [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


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

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