q = asin(54.44/72.17) = 0.843 radians. Note that this answer is not the same as that obtained via the scalar dot product. Rather, that answer is exactly pi (3.14159) - q.
Attribution:
Stephanie Redfern, Ranjeet (Ron) Agarwala, and Dr. Steve Gibbs. Mechanics I. The Saylor Academy 2014, http://www.saylor.org/courses/me102/