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?

A(1,2,−1), B(2,5,−2) and C(4,4,−3) are three vertices of a rectangle. What is the area of the rectangle?