Find a unit vector that is directed from the midpoint of the side AB to the midpoint of side
BC: The vector from the origin to the midpoint of AB is MAB = 1
2 (A+B) = 1
2 (−5ax+2az).
The vector from the origin to the midpoint of BC is MBC = 1
2 (B+C) = 1
2 (−3ax +ay −5az).
The vector from midpoint to midpoint is now MAB −MBC = 1
2 (−2ax − ay + 7az). The unit
vector is therefore