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Q.
The three planes $x+y=0, y+z=0$ and $x+z=0$
Three Dimensional Geometry
Solution:
The planes $x+y=0$
i.e. $x=-y$ and $y+z=0$
i.e. $z =- y$ meet in the line
$\frac{ x }{ l }=\frac{ y }{-1}=\frac{ z }{1}$.
Any point on this line is $( t ,- t , t )$.
This point lies in the plane $x + z =0$
if $t + t =0 \Rightarrow t =0$
So the three planes meet in a unique point $(0,0,0)$.