JEE Main & Advanced JEE Main Paper (Held On 15 April 2018) Slot-I

  • question_answer
    A variable plane passes through a fixed point \[(3,2,1)\] and meets \[x,y\] and \[z\] axes at A, B and C respectively. A plane is drawn parallel to \[yz-plane\] through \[A\], a second plane is drawn parallel \[zx-plane\] through \[B\] and a third plane is drawn parallel to \[xy-plane\] through C. Then the locus of the point of intersection of these three planes, is                                                 [JEE Online 15-04-2018]

    A) \[x+y+z=6\]  

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

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

    D)        \[\frac{1}{x}+\frac{1}{y}+\frac{1}{z}=\frac{11}{6}\]    

    Correct Answer: C

    Solution :

    Let \[a,b,c\] be the intercepts of the variable plane on the \[x,y,z\] axes respectively, then the equation of the plane is \[\frac{x}{a}+\frac{y}{b}+\frac{z}{c}=1\]. And the point of intersection of the \[xy,yz,zx\]planes parallel to the planes is clearly the point\[(a,b,c)\]. Since the point \[(3,2,1)\] lies on the variable plane, we have\[\frac{3}{a}+\frac{2}{b}+\frac{1}{c}=1\]. Thus the required locus is\[\frac{3}{x}+\frac{2}{y}+\frac{1}{z}=1\] So the answer is option C.


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