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Alik [6]
3 years ago
12

For the following exercises, use logarithmic differentiation to find dy/dx.

Mathematics
1 answer:
kondaur [170]3 years ago
8 0

If y=x^{\log_2(x)}, then taking the logarithm of both sides gives

\ln(y) = \ln\left(x^{\log_2(x)}\right) = \log_2(x) \ln(x)

Differentiate both sides with respect to x :

\dfrac{d\ln(y)}{dx} = \dfrac{d\log_2(x)}{dx}\ln(x) + \log_2(x)\dfrac{d\ln(x)}{dx}

\dfrac1y \dfrac{dy}{dx} = \dfrac{d\log_2(x)}{dx}\ln(x) + \dfrac{\log_2(x)}{x}

Now if z=\log_2(x), then 2^z=x. Rewrite

2^z = e^{\ln(2^z)} = e^{\ln(2)z}

Then by the chain rule,

\dfrac{d2^z}{dx} = \dfrac{dx}{dx}

\dfrac{de^{\ln(2)z}}{dx} = 1

e^{\ln(2)z} \ln(2) \dfrac{dz}{dx}= 1

\dfrac{dz}{dx} = \dfrac{1}{e^{\ln(2)z}\ln(2)}

\dfrac{dz}{dx} = \dfrac{1}{2^z \ln(2)}

\dfrac{d\log_2(x)}{dx} = \dfrac{1}{\ln(2)x}

So we have

\dfrac1y \dfrac{dy}{dx} = \dfrac{\ln(x)}{\ln(2)x}+ \dfrac{\log_2(x)}{x}

\dfrac1y \dfrac{dy}{dx} = \dfrac{\log_2(x)}{x}+ \dfrac{\log_2(x)}{x}

\dfrac1y \dfrac{dy}{dx} = \dfrac{2\log_2(x)}{x}

\dfrac1y \dfrac{dy}{dx} = \dfrac{\log_2(x^2)}{x}

\dfrac{dy}{dx} = \dfrac{y\log_2(x^2)}{x}

Replace y :

\dfrac{dy}{dx} = \dfrac{x^{\log_2(x)}\log_2(x^2)}{x}

\dfrac{dy}{dx} = x^{\log_2(x)-1}\log_2(x^2)

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