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Question: Answered & Verified by Expert
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 $y z$ - plane through $A$, a second plane is drawn parallel $z x$ plane through $B$ and a third plane is drawn parallel to $x y$ - plane through $C$. Then the locus of the point of intersection of these three planes, is
MathematicsThree Dimensional GeometryJEE MainJEE Main 2018 (15 Apr Shift 1 Online)
Options:
  • 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
    $\frac{1}{x}+\frac{1}{y}+\frac{1}{z}=\frac{11}{6}$
Solution:
2050 Upvotes Verified Answer
The correct answer is:
$\frac{3}{x}+\frac{2}{y}+\frac{1}{z}=1$
If $a, b, \mathrm{c}$ are 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 planes parallel to the $x y, y z$ and $z x$ planes is ( $a, b$, c).
As the point $(3,2,1)$ lies on the variable plane, so $\frac{3}{a}+\frac{2}{b}+\frac{1}{c}=1$
Therefore, the required locus is $\frac{3}{x}+\frac{2}{y}+\frac{1}{z}=1$

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