What are the direction cosines of a line perpendicular to the $yz$-plane?

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Coordinate planes divide the space into ........ octants.

If the direction cosines of two lines are such that $2l + m + 2n = 0$ and $3l^2 + 5m^2 - 11n^2 = 0$,then the angle between the two lines is

Let $P = (x_1, y_1, z_1)$ and $Q = (x_2, y_2, z_2)$ be two points. If the direction cosines of a line $AB$ are $l, m, n$,then the projection of the line segment $PQ$ on the line $AB$ is:

Let $ABC$ be a triangle with vertices at points $A(2, 3, 5)$,$B(-1, 3, 2)$ and $C(\lambda, 5, \mu)$ in three-dimensional space. If the median through $A$ is equally inclined with the axes,then $(\lambda, \mu)$ is equal to

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