If $\theta$ is the angle between the lines $AB$ and $CD$,then the projection of line segment $AB$ on line $CD$ is:

If a line has the direction ratios $-18, 12, -4,$ then what are its direction cosines?

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 ratios $(d.r.'s)$ of two lines are connected by the relations $a-b+c=0$ and $a^2-b^2+2c^2=0$, and $\theta$ is the angle between these lines, then $\cos \theta = $

If three mutually perpendicular lines have direction cosines $(l_1, m_1, n_1), (l_2, m_2, n_2)$ and $(l_3, m_3, n_3)$,then the line having direction cosines $(l_1 + l_2 + l_3), (m_1 + m_2 + m_3)$ and $(n_1 + n_2 + n_3)$ makes an angle of $...^o$ with each of the original lines.

The acute angle between the lines whose direction cosines are given by the equations $l+m+n=0$ and $2lm+2ln-mn=0$ is