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Repeated roots

  1. Feb 6, 2012 #1
    1. The problem statement, all variables and given/known data
    Given that 2 is a repeated root of the equation 2x^3 + px^2 + qx -4 = 0, find the third root and the values of p and q.


    2. Relevant equations
    b^2 - 4ac(?)


    3. The attempt at a solution
    Since it said 2 is a root I plugged in 2 as the value of x and rearranged the equation in terms of q, which I figured out to be q = -2(3+p), but when I plug that back into the original equation to try and find p, I got 16 + 4p - 12 - 4p - 4 = 0. The p's cancel out so I don't know if I think I've done something wrong.
     
  2. jcsd
  3. Feb 6, 2012 #2

    micromass

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    If you know about derivatives (which I assume you don't, since you posted in precalc): what do you know about the derivative if the original polynomial has a repeated root??

    If you do not know about derivatives: note that if 2 is a repeated root, then your original polynomial can be written as [itex]2(x-2)^2(x-c)[/itex] for a certain c. Work that out and compare it to the original polynomial.
     
  4. Feb 6, 2012 #3

    Mark44

    Staff: Mentor

    The OP posted in the calculus section, also, but I deleted it there. This seems to me to be a problem that doesn't require calculus.
     
  5. Feb 6, 2012 #4
    Where does c come from and what does "certain c" mean exactly?
     
  6. Feb 6, 2012 #5

    Mark44

    Staff: Mentor

    c is the third root that you're trying to find. "For certain c" means that we don't know exactly what that number is, but we know that the 3rd degree polynomial with a repeated root of 2 has to factor into 2(x - 2)2(x - c) for some real number c.
     
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