JEE Main & Advanced Mathematics Three Dimensional Geometry Question Bank Line and Plane

  • question_answer
    The equation of the straight line passing through (1, 2, 3) and perpendicular to the plane \[x+2y-5z+9=0\] is                     [MP PET 1991]

    A)            \[\frac{x-1}{1}=\frac{y-2}{2}=\frac{z-3}{-5}\]

    B)            \[\frac{x-1}{1}=\frac{y-2}{2}=\frac{z+5}{3}\]

    C)            \[\frac{x+1}{1}=\frac{y+2}{2}=\frac{z+3}{-5}\]

    D)            \[\frac{x+1}{1}=\frac{y+2}{2}=\frac{z-5}{3}\]

    Correct Answer: A

    Solution :

                       The line passes through point (1, 2, 3) is  \[\frac{x-1}{a}=\frac{y-2}{b}=\frac{z-3}{c}\] and it is perpendicular to the plane \[x+2y-5z+9=0,\] therefore the line must be \[\frac{x-1}{1}=\frac{y-2}{2}=\frac{z-3}{-5}\] because                    \[\sin \theta =\frac{1\,\,.\,\,1+2\,\,.\,\,2+(-\,5)\,\,(-\,5)}{\sqrt{{{1}^{2}}+{{2}^{2}}+{{5}^{2}}}\,.\,\,\sqrt{{{1}^{2}}+{{2}^{2}}+{{5}^{2}}}}=1\]                    \[\,\Rightarrow \,\,\theta ={{90}^{o}}\].


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