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Q.Find dydx\dfrac{dy}{dx} if

(i) y=log⁡(log⁡x)y = \log(\log x), x>1x > 1
(ii) y=sin⁡−1(1−x21+x2)y = \sin^{-1}\left(\dfrac{1-x^2}{1+x^2}\right), 0<x<10 < x < 1. OR If yx=xyy^x = x^y, find dydx\dfrac{dy}{dx}.
Assam AhsecAHSEC Higher Secondary (HS) Final Examination 2023Subjective· 4mImportance★★★★★
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(i) chain rule on log⁡(log⁡x)\log(\log x); (ii) substitute x=tan⁡θx=\tan\theta to simplify before differentiating; (OR) logarithmic differentiation on yx=xyy^x=x^y.

(i) y=log⁡(log⁡x)y = \log(\log x), x>1x>1:

By the chain rule, with u=log⁡xu = \log x:

dydx=1log⁡x⋅ddx(log⁡x)=1log⁡x⋅1x=1xlog⁡x.\frac{dy}{dx} = \frac{1}{\log x}\cdot\frac{d}{dx}(\log x) = \frac{1}{\log x}\cdot\frac1x = \frac{1}{x\log x}.

(ii) y=sin⁡−1(1−x21+x2)y = \sin^{-1}\left(\dfrac{1-x^2}{1+x^2}\right), 0<x<10<x<1:

Let x=tan⁡θx = \tan\theta, θ∈(0,π/4)\theta \in (0, \pi/4). Then

1−x21+x2=1−tan⁡2θ1+tan⁡2θ=cos⁡2θ=sin⁡(π2−2θ).\frac{1-x^2}{1+x^2} = \frac{1-\tan^2\theta}{1+\tan^2\theta} = \cos 2\theta = \sin\left(\frac{\pi}{2}-2\theta\right).

So y=sin⁡−1(sin⁡(π2−2θ))=π2−2θ=π2−2tan⁡−1xy = \sin^{-1}\left(\sin\left(\frac{\pi}{2}-2\theta\right)\right) = \frac{\pi}{2} - 2\theta = \frac{\pi}{2} - 2\tan^{-1}x (valid since π2−2θ∈(−π/2,π/2)\frac{\pi}{2}-2\theta \in (-\pi/2,\pi/2) for this range).

Differentiating: dydx=0−2⋅11+x2=−21+x2\dfrac{dy}{dx} = 0 - 2\cdot\dfrac{1}{1+x^2} = -\dfrac{2}{1+x^2}.


OR: yx=xyy^x = x^y. Find dydx\dfrac{dy}{dx}.

Take logarithm of both sides: xlog⁡y=ylog⁡xx\log y = y\log x.

Differentiate both sides with respect to xx (product rule on each side): …

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