JEE Main & Advanced Mathematics Three Dimensional Geometry Question Bank Sphere

  • question_answer
    If the plane \[2ax-3ay+4az+6=0\] passes through the midpoint of the line joining the centres of the spheres                   \[{{x}^{2}}+{{y}^{2}}+{{z}^{2}}+6x-8y-2z=13\]            and  \[{{x}^{2}}+{{y}^{2}}+{{z}^{2}}-10x+4y-2z=8\],  then \[a\] equals [AIEEE 2005]           

    A)            ? 2

    B)            2     

    C)            ? 1

    D)            1

    Correct Answer: A

    Solution :

                       \[{{S}_{1}}\equiv {{x}^{2}}+{{y}^{2}}+{{z}^{2}}+6x-8y-2z=13,\]\[{{C}_{1}}\equiv (-3,\,4,\,1)\,\]                    \[{{S}_{2}}\equiv {{x}^{2}}+{{y}^{2}}+{{z}^{2}}-10x+4y-2z=8,\]\[{{C}_{2}}\equiv (5,\,-2,\,1)\]                    So mid point of \[{{C}_{1}}{{C}_{2}}\] (say P)                        \[\equiv P\left( \frac{5-3}{2},\frac{4-2}{2},\frac{1+1}{2} \right)=P(1,\,1,\,1)\]            Now the plane \[2ax-3ay+4az+6=0\] passes through the point P,            So, \[2a(1)-3a(1)+4a(1)+6=0=2a-3a+4a+6=0\]            Þ \[3a+6=0\]Þ \[3a=-6\Rightarrow a=-2\].


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