You are on the shore of a canal of uniform width d and want to reach a point a distance L>d away along the other shore as quickly as possible. To achieve this, you first run along the shore at constant speed v1, then jump in the canal and swim directly toward your target at constant speed v2<v1, both being your maximum speeds. The water in the canal is motionless. The angle of your trajectory in the water with respect to the shore must obey:
(a) cosθ=v1/v2; (b) cosθ=v12/v22−1;
(c) cosθ=1−d/L; (d) cosθ=v2/v1.