JEE Main & Advanced Mathematics Differentiation Question Bank Differentiation of implicit function Parametric

  • question_answer
    If \[x=2\cos t-\cos 2t\],\[y=2\sin t-\sin 2t\], then at \[t=\frac{\pi }{4},\frac{dy}{dx}=\]

    A)          \[\sqrt{2}+1\]

    B)            \[\sqrt{2+1}\]

    C)            \[\frac{\sqrt{2+1}}{2}\]

    D)            None of these

    Correct Answer: A

    Solution :

               \[\frac{dx}{dt}=-2\sin t+2\sin 2t\] and \[\frac{dy}{dt}=2\cos t-2\cos 2t\]            Þ  \[\frac{dy}{dx}=\frac{\cos t-\cos 2t}{\sin 2t-\sin t}\]            Put \[t=\frac{\pi }{4},\] we have \[{{\left[ \frac{dy}{dx} \right]}_{t=\pi /4}}\]                    \[=\frac{\cos \pi /4-\cos \pi /2}{\sin \pi /2-\sin \pi /4}=\sqrt{2}+1\].


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