J & K CET Engineering J and K - CET Engineering Solved Paper-2012

  • question_answer
    Let \[P(x,y)\] be a point on the curve \[{{y}^{2}}=4x\] at which the tangent is perpendicular to the line \[2x+y=-2.\]Then, the coordinates of the point P are

    A)  \[(4,\,4)\]          

    B)  \[(4,\,\,-\,4)\]

    C)  \[(-4,\,\,4)\]         

    D)  \[(-4,\,\,-4)\]

    Correct Answer: A

    Solution :

    Given,  \[{{y}^{2}}=4x\] On differentiating w.r.t. x,  we get \[2y\frac{dy}{dx}=4\] \[\Rightarrow \] \[\frac{dy}{dx}=\frac{2}{y}\] Since, the tangent to the curve is perpendicular to the line \[2x+y=-2\] \[\therefore \] \[\frac{2}{y}\times (-2)=-1\] \[(\because \,\,\,{{m}_{1}}{{m}_{2}}=-1)\] \[\Rightarrow \] \[y=4\] \[\therefore \]  From Eq. (i), \[{{(4)}^{2}}=4x\,\Rightarrow x=4\] Hence, required point is \[(4,4)\].


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