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Homework Help: Horizontal tangent/point of inflection problem

  1. Nov 18, 2008 #1
    1. The problem statement, all variables and given/known data

    The function y=x(to the forth)+bx(squared)+8x+1 has a horizontal tangent and a point of inflection for the same value of x. What must be the value of b?

    2. Relevant equations

    I think you have to find the derivative, set it equal to 0 and solve for x

    3. The attempt at a solution
  2. jcsd
  3. Nov 18, 2008 #2


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    Homework Helper

    That's one thing you have to do alright. But the second derivative also has to be zero, doesn't it? I would suggest you solve the equation for the second derivative equal to zero for b and substitute it into the first equation. THEN solve for x.
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