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7. The radius of a circular disc is increasing at the rate of 3 cm/sec. Find the rate of change of the area of the disc the instant the area of the disc is 497 cm. 8. A man 6ft tall walks towards an 20ft light pole at the rate of 2 fusec. a) find the rate the length of his shadow increasing when he is 7 ft from the pole. b) find the rate at what rate the tip of his shadow moving when he is 8ft from the pole 9. A balloon rising from the ground at the rate of 80 fu/min is tracked by a rangefinder at pt A located 200 ft from the balloon's lift off point. Find the rate at which angle A and range r are changing the instant the balloon is 150 ft above the ground. 10. Train A leaves the station travelling north at a rate of 36 mi/hr. At the same time train B leaves the station traveling east at a rate of 48 mi/hr. At what rate is the distance between then changing after 25 mins? 1 1. As a spherical snowball melts, its radius decreases at a rate of 2cm/min. Find the rate at which the surface area decreases the instant the diameter is 10 cm. 12. Sand falling off a conveyor belt at the rate of 15 cu ft/ min is forming a conical pile. If the height of the pile is 3 times its radius, find the rate the height of the pile of sand is changing the instant its radius is 6 ft