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If y=(logx)(logx)y = (log x)^{(log x)}, x>1x > 1 then dydx\frac{dy}{dx} is equal to

Solution

✅ Correct Option: 4

Given y=(log⁡x)log⁡xy = (\log x)^{\log x} where x>1x > 1.

Taking log on both sides:

log⁡y=log⁡[(log⁡x)log⁡x]\log y = \log[(\log x)^{\log x}]

log⁡y=(log⁡x)⋅log⁡(log⁡x)\log y = (\log x) \cdot \log(\log x)


Differentiating both sides with respect to xx:

ddx(log⁡y)=ddx[(log⁡x)⋅log⁡(log⁡x)]\frac{d}{dx}(\log y) = \frac{d}{dx}[(\log x) \cdot \log(\log x)]

1y⋅dydx=(log⁡x)⋅ddx[log⁡(log⁡x)]+log⁡(log⁡x)⋅ddx[log⁡x]\frac{1}{y} \cdot \frac{dy}{dx} = (\log x) \cdot \frac{d}{dx}[\log(\log x)] + \log(\log x) \cdot \frac{d}{dx}[\log x]


Finding the derivatives:

ddx[log⁡(log⁡x)]=1log⁡x⋅1x=1xlog⁡x\frac{d}{dx}[\log(\log x)] = \frac{1}{\log x} \cdot \frac{1}{x} = \frac{1}{x \log x}

ddx[log⁡x]=1x\frac{d}{dx}[\log x] = \frac{1}{x}


Substituting back:

1y⋅dydx=(log⁡x)⋅1xlog⁡x+log⁡(log⁡x)⋅1x\frac{1}{y} \cdot \frac{dy}{dx} = (\log x) \cdot \frac{1}{x \log x} + \log(\log x) \cdot \frac{1}{x}

1y⋅dydx=1x+log⁡(log⁡x)x\frac{1}{y} \cdot \frac{dy}{dx} = \frac{1}{x} + \frac{\log(\log x)}{x}

1y⋅dydx=1+log⁡(log⁡x)x\frac{1}{y} \cdot \frac{dy}{dx} = \frac{1 + \log(\log x)}{x}


dydx=y⋅1+log⁡(log⁡x)x\frac{dy}{dx} = y \cdot \frac{1 + \log(\log x)}{x}

Substituting y=(log⁡x)log⁡xy = (\log x)^{\log x}:

dydx=(log⁡x)log⁡x[1+log⁡(log⁡x)x]\frac{dy}{dx} = (\log x)^{\log x} \left[\frac{1 + \log(\log x)}{x}\right]

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