Final answer:
To determine the athlete's average velocities in different legs of the swim, we need to calculate the total distance covered and the total time taken. The average velocity in the first half of the swim is l/t1, in the second half it is also l/t1, and for the round trip it is (2l)/(t1 + t2).
Step-by-step explanation:
To determine the athlete's average velocities, we first need to calculate the total distance covered and the total time taken for each leg of the swim. In the first half of the swim, the athlete covers a distance of l/2 in a time of t1/2, so the average velocity in the first half is (l/2)/(t1/2) = l/t1. In the second half of the swim, the athlete covers the same distance of l/2 in the same time of t1/2, so the average velocity in the second half is also l/t1. For the round trip, the total distance covered is l + l = 2l and the total time taken is t1 + t2. Therefore, the average velocity for the round trip is (2l)/(t1 + t2).