The line AB is a common perpendicular to to two skew lines AP and BQ, and C and R are the midpoints of AB and PQ respectively. Prove by vector methods, that CR and AB are perpendicular.
I tried the vectors, AP=s, AB=b, BQ=c, and PQ=d
a.b=0, and b.c=0
this would equal 0 if b.d is equal to one but I don't see how that relationship comes about.