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Inverse function of x^3 + 1

  1. Jun 4, 2004 #1
    I was just doing a problem in a review book and my answer doesnt match with what the books says.

    It f(x)=x^3+1 and if f^-1 is the inverse function of f, what is f^-1(4)?

    I got an answer of 0.02 but the book says it is 1.44.

    All I did was sub in 4 into the equation, which I get 64, then set that under one...1/64 and get 0.02 as the inverse. How did the book get 1.44?
  2. jcsd
  3. Jun 4, 2004 #2
    You're mistaking the inverse with the reciprocal.

    The problem is asking for what value of x will satisfy f(x) = 4.

    Clearly f(.002) does not equal 4, but 4(1.44) = 1.44^3 + 1 = 3.985984 ~ 4

  4. Jun 4, 2004 #3
    The notation is what really confuses people most of the time. The inverse of a function basically switches the x and y values. If you want to find the inverse for a function than all you would have to do is switch the values. In other words, replace all the x's with y's and vice versa. Than solve for y (assuming you originally had a y= equation) and you will have your inverse function. Than you can just plug your value in. In this case it would be:
    y = x^3 + 1
    For the inverse:
    x = y^3 + 1
    y = (x - 1) ^ (1/3)
    so f^-1(4) = (4 - 1) ^ (1/3) = 3 ^ (1/3) ~ 1.44
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