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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=(logx)logxy = (\log x)^{\log x} where x>1x > 1.

Taking log on both sides:

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

logy=(logx)log(logx)\log y = (\log x) \cdot \log(\log x)


Differentiating both sides with respect to xx:

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

1ydydx=(logx)ddx[log(logx)]+log(logx)ddx[logx]\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(logx)]=1logx1x=1xlogx\frac{d}{dx}[\log(\log x)] = \frac{1}{\log x} \cdot \frac{1}{x} = \frac{1}{x \log x}

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


Substituting back:

1ydydx=(logx)1xlogx+log(logx)1x\frac{1}{y} \cdot \frac{dy}{dx} = (\log x) \cdot \frac{1}{x \log x} + \log(\log x) \cdot \frac{1}{x}

1ydydx=1x+log(logx)x\frac{1}{y} \cdot \frac{dy}{dx} = \frac{1}{x} + \frac{\log(\log x)}{x}

1ydydx=1+log(logx)x\frac{1}{y} \cdot \frac{dy}{dx} = \frac{1 + \log(\log x)}{x}


dydx=y1+log(logx)x\frac{dy}{dx} = y \cdot \frac{1 + \log(\log x)}{x}

Substituting y=(logx)logxy = (\log x)^{\log x}:

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

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