THREE DIMENSIONAL GEOMETRY

® Angle between skew lines is the angle between two intersecting lines drawn from any point (preferably through the origin) parallel to each of the skew lines. ® If l , m , n and l , m , n are the direction cosines of two lines; and θ is the acute angle between the two lines; then cos θ = | l l + m m + n n | ® If a , b , c and a , b , c are the direction ratios of two lines and θ is the acute angle between the two lines; then cos θ = a a c c