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If x=3sin⁡θ−sin⁡3θx = 3\sin\theta - \sin 3\theta, y=3cos⁡θ−cos⁡3θy = 3\cos\theta - \cos 3\theta, then d2ydx2\frac{d^2y}{dx^2} at θ=π3\theta = \frac{\pi}{3} is

Solution

✅ Correct Option: 1

dydx=3sin⁡3θ−3sin⁡θ3cos⁡θ−3cos⁡3θ=2cos⁡2θsin⁡θ2sin⁡2θsin⁡θ=cot⁡2θ\frac{dy}{dx}=\frac{3\sin 3\theta-3\sin\theta}{3\cos\theta-3\cos 3\theta}=\frac{2\cos 2\theta\sin\theta}{2\sin 2\theta\sin\theta}=\cot 2\theta. Then d2ydx2=−2csc⁡22θdx/dθ\frac{d^2y}{dx^2}=\frac{-2\csc^2 2\theta}{dx/d\theta}. At θ=π3\theta=\frac{\pi}{3}, csc⁡22π3=43\csc^2\frac{2\pi}{3}=\frac43 and dxdθ=92\frac{dx}{d\theta}=\frac92, giving −8/39/2=−1627\frac{-8/3}{9/2}=-\frac{16}{27}.

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