# Thread: Angle between lines in 3d

1. ## Angle between lines in 3d

Find the angle between the lines whose direction cosines are connected by the relations

l+m+n=0, l^2+m^2+n^2=0

2. ## Re: Angle between lines in 3d

If l,m,n are real then they must all equal 0 since x^2 >= 0 for all real x.