Derivative of (ln x)^ln x

Calculus Level 2

Evaluate the derivative of

f ( x ) = ( ln x ) ln x f(x)=(\ln x)^{\ln x}

at x = e , x = e, where ln x \ln x denotes the logarithm of base e e (Euler constant).

0 0 1 e \frac{1}{e} 1 1 e e

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1 solution

Adam Ardeishar
Oct 18, 2015

Let y = f ( x ) y=f(x) . Then ln y = ln x ln ln x \ln{y}=\ln{x} \ln{\ln{x}} by logarithm rules. Differentiating implicitly, using product rule and chain rule, we get 1 y d y d x = 1 x ln ln x + ln x 1 ln x 1 x \frac{1}{y} \cdot \frac{dy}{dx} = \frac{1}{x} \cdot \ln{\ln{x}} +\ln{x} \cdot \frac{1}{\ln{x}} \cdot \frac{1}{x} We know that y = f ( e ) = 1 1 = 1 y= f(e) = 1^1 =1 so plugging everything in we get 1 1 d y d x = 0 + 1 1 1 e d y d x = 1 e \frac{1}{1} \cdot \frac{dy}{dx}=0+1 \cdot 1 \cdot \frac{1}{e} \longrightarrow \frac{dy}{dx} = \boxed{\frac{1}{e}}

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