An athlete leaves one end of a pool of length l at t = 0 and arrives at the other end at time t1. she swims back and arrives at the starting position at time t2. if she is swimming initially in the positive x direction, determine her average velocities symbolically in the first half of the swim, the second half of the swim, and the round trip. (assume that time t2 is from the other end of the pool to the starting point. use any variable or symbol stated above as necessary. indicate the direction with the sign of your answer.)