Find the angle between the lines whose d.cs $l, m, n$ are connected by the relation, $3l+m+ 5n = 0$ and $6mn-2nl+5lm=0$.

Show that the measures of the angles between the four diagonals of a rectangular parallelepiped whose edges are $a, b, c$ are

${\cos }^{-1}\left(\frac{a^2\pm b^2\pm c^2}{a^2+b^2+c^2}\right)$.

State, which of the following statements are true (T) or false (F)

Through any four points one and only one plane can pass.

State, which of the following statements are true (T) or false (F)

The equation of $xy-$plane is $x+y=0$.

State the statement is true (T) or false (F)

The plane $ax+by+c=0$ is perpendicular to $z-$axis.

State whether the following statement is true (T) or false (F).

The equation of the plane parallel to $xz-$plane and passing through $(2, -4, 0)$, is $\mathrm{y}+4=0$.

State, whether the following statement is true (T) or false (F).

The planes $2\mathrm{x}-\mathrm{y}+\mathrm{z}-1=0$, and $6\mathrm{x}-3\mathrm{y}+3\mathrm{z}=1$, are coincident. 

State, whether the following statement is true (T) or false (F).

The planes $2\mathrm{x}+4\mathrm{y}-\mathrm{z}+1=0$, and $\mathrm{x}-2\mathrm{y}-6\mathrm{z}+3=0$, are perpendicular to each other.