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JEE Main & Advanced Mathematics Three Dimensional Geometry Question Bank System of Co-ordinates, Direction cosines and direction ratios, Projection

  • question_answer
    The projection of a line on a co-ordinate axes are 2, 3, 6. Then the length of the line is [Orissa JEE 2002]

    A)            7

    B)            5

    C)            1

    D)            11

    Correct Answer: A

    Solution :

               Let d be the length of line, then projection on x-axis = dl = 2, projection on y-axis = dm = 3, Projection on  z-axis = dn = 6            Now \[{{d}^{2}}({{l}^{2}}+{{m}^{2}}+{{n}^{2}})=4+9+36\]            Þ \[{{d}^{2}}(1)=49\] Þ \[d=7\].


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