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Inflection point problem

  1. Oct 24, 2013 #1
    1. The problem statement, all variables and given/known data
    Determine A and B so that y = Ax^(1/9) + Bx(-1/9) has an inflection point as (1/9)


    2. Relevant equations

    A + B = 9

    3. The attempt at a solution

    F' = Ax^(-8/9)/9 - Bx^(-10/9)/9
    F'' = -8Ax^(---)/81 + 10Bx^(---)/81

    -8A + 10B = 0
    A + B = 9
    -10(9-A) - 8A = 0
    -90 - 18A = 0
    A = -90/18
    B = 14

    But, this is not correct.. I don't understand what I did wrong?
     
  2. jcsd
  3. Oct 24, 2013 #2

    vela

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    I'm assuming you meant "at the point (1,9)" not "as (1/9)".

    Check your algebra when solving for A and B. You made a few sign mistakes.
     
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