If x=$acos^3\theta$ and y=$asin^3\theta,$then find the value of $\Large \frac{d^2y}{dx^2}$ at $ \theta=\Large \frac{\pi}{6}$.

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(Note: This question has been split into 2 questions) This question appeared in 65-1,65-2 and 65-3 versions of the paper in 2013.