Let the foot of perpendicular from a point P (1,2,-1) to the straight line L: x/1=y/0=z/-1 be N. Let a line be drawn from P parallel to the plane…

Let the foot of perpendicular from a point $\displaystyle \mathrm{P}(1,2,-1)$ to the straight line $\displaystyle \mathrm{L}: \frac{x}{1}=\frac{y}{0}=\frac{z}{-1}$ be N. Let a line be drawn from P parallel to the plane $\displaystyle x+y+2 z=0$ which meets L at point Q. If $\displaystyle \alpha$ is the acute angle between the lines $\displaystyle P N$ and $\displaystyle P Q$, then $\displaystyle \cos \alpha$ is equal to \_\_\_\_.