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64. DEPRECIATION The value I (in thousands of dollars) of an industrial machine is modeled by 3N + 430 2/3 V(N) = N+1 where N is the number of hours the machine is used each day. Suppose further that usage varies with time in such a way that N(D = Vt - 10t + 61 where t is the number of months the machine has been in operation. a. Over what time interval is the value of the machine increasing? When is it decreasing? b. At what time t is the value of the machine the largest? What is this maximum value