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BCECE Engineering BCECE Engineering Solved Paper-2011

  • question_answer
    If the line \[x-1=0\]is the directrix of parabola \[{{y}^{2}}-kx+8=0,\] then one of the value of \[k\]is

    A)  1/8                                        

    B)         8                            

    C)         4                                            

    D)         1/4

    Correct Answer: C

    Solution :

    Given parabola is                           \[{{y}^{2}}-kx+8=0\]                 \[\Rightarrow \]    \[{{y}^{2}}=kx-8=k(x-8/k)=4AX\] where \[4\,A=k\] Directrix of the parabola is \[X+A=0\] Comparing it with \[x-1=0,\]we get                                 \[1=\frac{8}{k}-\frac{k}{4}\]                 \[\Rightarrow \]    \[4k=32-{{k}^{2}}\]                 \[\therefore \]      \[k=-8,4\]


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