Derivatives of Inverse Functions

Find derivatives of inverse trig functions and general inverse functions for AP Calculus AB.

Inverse function derivatives appear on the AP exam both as a concept and with specific inverse trig functions.

General Inverse Function Derivative

If g=f1g = f^{-1}, then:

g(x)=1f(g(x))g'(x) = \frac{1}{f'(g(x))}

Inverse Trig Derivatives

ddx[arcsinx]=11x2\frac{d}{dx}[\arcsin x] = \frac{1}{\sqrt{1-x^2}}

ddx[arccosx]=11x2\frac{d}{dx}[\arccos x] = \frac{-1}{\sqrt{1-x^2}}

ddx[arctanx]=11+x2\frac{d}{dx}[\arctan x] = \frac{1}{1+x^2}

With chain rule: ddx[arctanu]=u1+u2\frac{d}{dx}[\arctan u] = \frac{u'}{1+u^2}.

Worked Example

f(x)=x3+xf(x) = x^3 + x. f(2)=10f(2) = 10. Find (f1)(10)(f^{-1})'(10).

f(x)=3x2+1f'(x) = 3x^2 + 1. f(2)=13f'(2) = 13.

(f1)(10)=1f(2)=113(f^{-1})'(10) = \frac{1}{f'(2)} = \frac{1}{13}.

Practice Problems

    1. ddx[arcsin(3x)]\frac{d}{dx}[\arcsin(3x)].
    1. f(x)=2x+cosxf(x) = 2x + \cos x. Find (f1)(1)(f^{-1})'(1) given f(0)=1f(0) = 1.

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Key Takeaways

  • (f1)(b)=1f(a)(f^{-1})'(b) = \frac{1}{f'(a)} where f(a)=bf(a) = b.

  • Know the three inverse trig derivatives.

  • Apply chain rule for compositions.

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