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8. The amount of a drug {in mg) that remains in the bloodstream t hours after an injection is modeled byA(t) = 200(0.75)'. At what rate is the amount of the drug changing after 4 hours? Be sure to include units in your answer. 9. The water depth (in feet} in a harbor can be modeled by D(t) = 7 + 6 cos[.5{}3(t 6.75)], where t is the number of hours after midnight on a particular day. a. At what rate is the water depth changing at 10 am on that day? b. Is the tide coming in or going out at 10 am? Explain. 10. A 4-foot tall boy walks away from a 15-foot tall streetlight at a rate of 3 ft/sec. Find the rate at which the length of his shadow is changing. Is the shadow getting longer or shorter