1. The problem statement, all variables and given/known data Two particles are moving on trajectories given by r=a+ut and r=b+vt respectively where a, b, u and v are constant vectors and t is the time. Show that the particles will collide if v.(bxu)=v.(axu) Obtain an expression for the time of the collision in terms of a, b, u and v. (I think I've done the parts so far.) Hence, or otherwise, show that the collision will take place at position r=b+((a.(bxu))/(v.(bxu))v 2. Relevant equations If three vectors a, b, c are linearly dependent, a.(bxc)=o 3. The attempt at a solution a+ut=b+vt ut=(b-a)+vt ut, vt, and b-a are coplanar. Since t is a scalar multiple, u, v, and b-a are coplanar. v.((b-a)xu)=0 v.(bxu-axu)=o v.(bxu)=v.(axu) ut-vt=b-a (u-v)t=b-a t=mod(b-a)/mod(u-v) I can't see how this expression for to is equivalent to ((a.(bxu))/(v.(bxu)), so I'm stuck.