$A$ line makes angles $\alpha, \beta, \gamma$ with the coordinate axes,then $\cos 2\alpha + \cos 2\beta + \cos 2\gamma$ is equal to

Direction ratios of two lines are $a, b, c$ and $\frac{1}{bc}, \frac{1}{ca}, \frac{1}{ab}$. The lines are

Consider the following statements:
Assertion $(A)$: The direction ratios of a line $L_1$ are $2, 5, 7$ and the direction ratios of another line $L_2$ are $\frac{4}{\sqrt{19}}, \frac{10}{\sqrt{19}}, \frac{14}{\sqrt{19}}$. Then the lines $L_1, L_2$ are parallel.
Reason $(R)$: If the direction ratios of a line $L_1$ are $a_1, b_1, c_1$,the direction ratios of a line $L_2$ are $a_2, b_2, c_2$ and $a_1 a_2 + b_1 b_2 + c_1 c_2 = 0$,then the lines $L_1, L_2$ are parallel. Which one of the following is true?

If the direction cosines of two lines are given by $l+m+n=0$ and $l^2-5m^2+n^2=0$,then the angle between them is

If the projections of a line on the coordinate axes are $2, -1, 2$,then the length of the line is

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