13. s(t) = 2t4 8t + 8t + 1 is the position function of a body moving along a coordinate line, where t 0, and s(t) is measured in feet and t in seconds. a. Determine the times when the body is stationary. b. When is the body moving in the positive direction? c. When is the body moving in the negative direction? 14. The position of a particle from the origin of an axis at time t is given by s(t) = 3t - t, t 0 where t is in second and s is in feet. a. When is the particle at rest? b. Find the total distance traveled during the first 5 seconds. C. At what time interval is the particle speeding up? 15. For x2y = 8, find dy dt dx when x = 2, y = 2, = 3. dt 16. The volume V of a cube with sides of length x inches is changing with respect to time t (in seconds). When the sides of the cube are 10 in. long and increasing at the rate of 0.5 in/sec, how fast is the volume of the cube increasing? - 17. A point moves along the curve 2x2 y2 = 2. When the point is at (3,-4), its x-coordinate is increasing at the rate of 2 units per second. How fast is its y-coordinate changing at that instant of time? 18. In calm waters, the oil spilling from the ruptured hull of a grounded tanker spreads in all directions. Assuming that the polluted area is circular, determine how fast the area is increasing when the radius of the circle is 60 ft and is increasing at the rate of ft/sec?