69. Newton's law of gravitational attraction states that if two bodies are at a distance r apart, then the force F exerted by one body on the other is given by F(r) = k p2 where k is a positive constant. Suppose that, as a function of time, the distance between the two bodies is given by r(t) = 49t 4.9t. 0 t 10. (a) Find the rate of change of F with respect to t. (b) Show that (F r)'(3) = (For)'(7).