an athlete leaves one end of a pool pf length l at t=0 and arrives at the other end at the time t1. she swim back and arrived at the starting position at the time t2. if she is swimming initially in the position x direction. determine her average velocities symbolically in (a) the first half of the swim. (b) the second half of the swim and (c) the round trip (d) what is her average speed for the round trip?