Math grade 12
posted by Anonymous .
the function f(x)= o.005t^2+0.8t+12 models the relationship between a certain vehicleâ€™s speed and fuel economy, where is the fuel economy in kilometres per litre and x is the speed of the vehicle in kilometres per hour. Determine the rate of change in fuel economy for 10 km/h intervals in speed, and use your results to determine the speed that gives the best fuel economy.
ANSWER : 18km/h
i confused i don't understand how to solve this problem please help

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