A motorboat cuts its engine when its speed is 10. 0m/s and then coasts to rest. The equation describing the motion of the motorboat during this period is v=vi e^⁻ct, where v is the speed at time t, vi is the initial speed at t=0 , and c is a constant. At t=20. 0s , the speed is 5. 00m/s (c) Differentiate the expression for v(t) and thus show that the acceleration of the boat is proportional to the speed at any times