The reflection of the point $(\alpha, \beta, \gamma)$ in the $XY$-plane is:

Find the locus of a point such that the sum of its distances from the $xy$-plane and the $yz$-plane is equal to its distance from the $zx$-plane.

If the direction cosines of two lines are such that $l+m+n=0$ and $l^2+m^2-n^2=0$,then the angle between them is

Find the direction cosines of the vector joining the points $A(1, 2, -3)$ and $B(-1, -2, 1)$,directed from $A$ to $B$.

The angle between the lines whose direction ratios satisfy the equations $l+m+n=0$ and $l^2=m^2+n^2$ is

If the line with direction ratios $(1, \alpha, \beta)$ is perpendicular to the line with direction ratios $(-1, 2, 1)$ and parallel to the line with direction ratios $(\alpha, 1, \beta)$,then $(\alpha, \beta)$ is