21. Let s(t) = t/(t + 5) be the position function of a particle moving along a coordinate line, where s is in meters and t is in seconds. Use a graphing utility to generate the graphs of s(t), v(t), and a(t) for t > 0, and use those graphs where needed. (a) Use the appropriate graph to make a rough estimate of the time at which the particle first reverses the direction of its motion; and then find the time exactly. (b) Find the exact position of the particle when it first re- verses the direction of its motion. (c) Use the appropriate graphs to make a rough estimate of the time intervals on which the particle is speeding up and on which it is slowing down; and then find those time intervals exactly