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