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Worked solution

A variable base and exponent

Find dydx\dfrac{dy}{dx} given that y=xxy=x^{\sqrt{x}} where x>0x>0.

CJC · Discussion · Question 5

Answer

dydx=xx(2+ln⁡x)2x\dfrac{dy}{dx}=\dfrac{x^{\sqrt{x}}(2+\ln x)}{2\sqrt{x}}, x>0x>0.

Logarithmic differentiation

Take natural logarithms: ln⁡y=xln⁡x\ln y=\sqrt{x}\ln x. Differentiate using the product rule: 1ydydx=x⋅1x+ln⁡x⋅12x−1/2=1x+ln⁡x2x\dfrac1y\dfrac{dy}{dx}=\sqrt{x}\cdot\dfrac1x+\ln x\cdot\dfrac12x^{-1/2}=\dfrac1{\sqrt{x}}+\dfrac{\ln x}{2\sqrt{x}}. Multiply by y=xxy=x^{\sqrt{x}} to obtain dydx=xx(2+ln⁡x)2x\dfrac{dy}{dx}=\dfrac{x^{\sqrt{x}}(2+\ln x)}{2\sqrt{x}}.

How did this part go?

Common mistakes

The exponent is √x, not x. Both factors in √x ln x vary with x.

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