BCECE Engineering BCECE Engineering Solved Paper-2004

  • question_answer
    If \[x=(cos\theta +\theta sin\theta )\]  \[y=a(sin\theta -\theta cos\theta ),\]then\[\frac{dy}{dx}\]is equal to:

    A) \[\cos \theta \]    

    B)                         \[\tan \theta \]               

    C)        \[\sec \theta \]                

    D)        \[\cos ec\,\theta \]

    Correct Answer: B

    Solution :

    \[x=a(cos\theta +\theta sin\theta )\]and \[y=a(sin\theta -\theta cos\theta )\] On differentiating w.r.t\[\theta ,\]respectively \[\frac{dx}{d\theta }=a(-sin\theta +sin\theta +\theta \,cos\theta )\] \[=a\theta \cos \theta \] and \[\frac{dy}{d\theta }=a(cos\theta +\theta sin\theta -cos\theta )\] \[=a\,\theta \sin \theta \]                 \[\therefore \]  \[\frac{dy}{dx}=\frac{dy/d\theta }{dx/d\theta }=\frac{a\theta \sin \theta }{a\theta \cos \theta }\] \[=\tan \theta \]


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