Note that f(x) as given is not invertible. By definition of inverse function,
[tex]f\left(f^{-1}(x)\right) = x[/tex]
[tex]\implies f^{-1}(x)^3 - f^{-1}(x) + 2 = x[/tex]
which is a cubic polynomial in [tex]f^{-1}(x)[/tex] with three distinct roots, so we could have three possible inverses, each valid over a subset of the domain of f(x).
Choose one of these inverses by restricting the domain of f(x) accordingly. Since a polynomial is monotonic between its extrema, we can determine where f(x) has its critical/turning points, then split the real line at these points.
f'(x) = 3x² - 1 = 0 ⇒ x = ±1/√3
So, we have three subsets over which f(x) can be considered invertible.
• (-∞, -1/√3)
• (-1/√3, 1/√3)
• (1/√3, ∞)
By the inverse function theorem,
[tex]\left(f^{-1}\right)'(b) = \dfrac1{f'(a)}[/tex]
where f(a) = b.
Solve f(x) = 2 for x :
x³ - x + 2 = 2
x³ - x = 0
x (x² - 1) = 0
x (x - 1) (x + 1) = 0
x = 0 or x = 1 or x = -1
Then [tex]\left(f^{-1}\right)'(2)[/tex] can be one of
• 1/f'(-1) = 1/2, if we restrict to (-∞, -1/√3);
• 1/f'(0) = -1, if we restrict to (-1/√3, 1/√3); or
• 1/f'(1) = 1/2, if we restrict to (1/√3, ∞)
