[8] 5. A circle is growing with time. a) At a certain moment, the circle's radius is 20 inches and is increasing at 3 in/sec. At what rate is the circle's area increasing at this same moment? b) At another moment, the circle's area is increasing at 100 in?/sec and its radius is increasing at 2 in/sec. At this moment, what is the radius of the circle? [6) 6. A particle is travelling along the circular path z? + y? = 100. At the moment when the particle passes through the point (x, y) = (8, 6), its horizontal velocity is dx/dt = -15 meters per second. What is the particle's vertical velocity dy/dt at that same moment