JEE Main & Advanced JEE Main Paper (Held On 9 April 2017)

  • question_answer
    If \[y=mx+c\] is the normal at a point on the parabola \[{{y}^{2}}=8x\] whose focal distance is 8 units, then |c| is equal to: [JEE Online 09-04-2017]

    A)  \[8\sqrt{3}\]                     

    B)  \[10\sqrt{3}\]

    C)  \[2\sqrt{3}\]                     

    D)  \[16\sqrt{3}\]

    Correct Answer: B

    Solution :

                    \[c=-29m-9{{m}^{3}}\] \[a=2\]                 Given    \[{{(a{{t}^{2}}-a)}^{2}}+4{{a}^{2}}{{t}^{2}}=64\] \[\to (a({{t}^{2}}+1))=8\] \[\Rightarrow \,{{t}^{2}}+1=4={{t}^{2}}=3\] \[\Rightarrow t=\sqrt{3}\] \[\therefore \,\,C=-a\,[-2t\,-{{t}^{3}}]\,=2at\,(2+{{t}^{2}})\] \[=2\sqrt{3}\,(5)\] \[|C|\,=10\sqrt{3}\]


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