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Q7 a) The annual cost C, in £ millions, of pumping water at a hydroelectric power station is given by C = r + 25/4r + 6, where r is the radius of the supply pipe in meters. Determine the radius of pipe which minimises the annual cost. (8)

b) The temperature T℃ at a certain location, t hours after midday is given by

T = t³/3 - 42 + 15t + 12

Using differential calculus, determine when the temperatures start’s to fall. (8)