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Q.If y=sin⁡−1(3x−4x3)y = \sin^{-1}(3x - 4x^3) then dydx=\dfrac{dy}{dx} =

(a) 31−x2\dfrac{3}{\sqrt{1-x^2}}
(b) −41−x2\dfrac{-4}{\sqrt{1-x^2}}
(c) 31+x2\dfrac{3}{\sqrt{1+x^2}}
(d) −31+x2\dfrac{-3}{\sqrt{1+x^2}}
Jharkhand JacJAC Intermediate Board 2025MCQ· 1mImportance★★★★★
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Recognise 3x - 4x³ as the triple-angle expansion of sine, which collapses the inverse to 3sin⁻¹x.

Recall the identity sin⁡3θ=3sin⁡θ−4sin⁡3θ\sin 3\theta = 3\sin\theta - 4\sin^3\theta.

Let x=sin⁡θx = \sin\theta, i.e. θ=sin⁡−1x\theta = \sin^{-1}x. Then 3x−4x3=3sin⁡θ−4sin⁡3θ=sin⁡3θ3x - 4x^3 = 3\sin\theta - 4\sin^3\theta = \sin3\theta.

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