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Homework Help: Derivative of an inverse for Calc 1

  1. Oct 19, 2013 #1
    1. The problem statement, all variables and given/known data
    Find (f−1)'(a).
    f(x) = 5x^3 + 3x^2 + 5^x + 4, a = 4

    2. Relevant equations
    I'm not entirely sure but I assume I have to use d/dx(f-1) = 1/f '(f-1(x))


    3. The attempt at a solution
    So far I switched y and x. Found dx/dy to be 15y^2 + 6y + 5. Then I switched dx/dy to dy/dx so the answer became 1/15^2 + 6y +5 and I plugged in 4 to get 1/269. But the answer is wrong
     
  2. jcsd
  3. Oct 19, 2013 #2
    No, do not plug in 4 solve the equation being equal to 4.
     
  4. Oct 19, 2013 #3
    Ok so I set 1/15y^2 + 6y + 5 equal to 4. Ended up with 1 = 4(15y^2 + 6y + 5) so 1 = 60y^2 + 24y +20. Do I factor it from here?
     
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