The projections of a line on the coordinate axes are $2, 3, 6$. Then the length of the line is

Which of the following conditions is satisfied by a point lying on the $z$-axis?

$E(1,0,0), F(0,2,0), G(0,0,3)$ are respectively the mid-points of the sides $AB, BC, CA$ of $\triangle ABC$. If $a_1, b_1, c_1$ and $a_2, b_2, c_2$ are respectively the direction ratios of $AF$ and $BG$, then $\frac{a_1^2+b_1^2+c_1^2}{a_2^2+b_2^2+c_2^2}=$

The number of straight lines that are equally inclined to the three-dimensional coordinate axes is

The direction ratios of the diagonal of a cube that joins the origin to the opposite corner are (when the $3$ concurrent edges of the cube are the coordinate axes):

Find the angle between two lines whose direction cosines are given by the relations $l + m + n = 0$ and $l^2 + m^2 - n^2 = 0$.