The equation of the plane passing through the intersection of the planes 2x + y + 2z = 9, 4x $$-$$ 5y $$-$$ 4z = 1 and the point (3, 2, 1) is

The distance between the parallel planes $$4x - 2y + 4z + 9 = 0$$ and $$8x - 4y + 8z + 21 = 0$$

What are the direction cosines of Z-axis?

If a line in 3 dimensions makes angles α, β and γ with the positive directions of the coordinate axes, then what is $\cos (\alpha +\beta )\cos (\alpha -\beta )$ equal to?