JEE Main & Advanced Mathematics Applications of Derivatives Question Bank Self Evaluation Test - Application of Derivatives

  • question_answer
    The equation of one of the tangents to the curve \[y=\cos (x+y),-2\pi \le x\le 2\pi \] that is parallel to the line \[x+2y=0,\] is

    A) \[x+2y=1\]

    B) \[x+2y=\pi /2\]

    C) \[x+2y=\pi /4\]

    D) None of these

    Correct Answer: B

    Solution :

    [b] \[y=\cos (x+y)....(1)\] \[\therefore \frac{dy}{dx}=-\sin (x+y)\left\{ 1+\frac{dy}{dx} \right\}\]             \[=-\frac{\sin (x+y)}{1+\sin (x+y)}=-\frac{1}{2}\] \[\Rightarrow \sin (x+y)=1,\] so \[\cos (x+y)=0\] \[\therefore \] from \[(1)y=0\] and \[(x+y)=2n\pi +\frac{\pi }{2}\] Tangent at \[\left( \frac{\pi }{2},0 \right)\] is \[x+2y=\frac{\pi }{2}\]


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