General: Fuel Economy The gas mileage (in miles per gallon) of a subcompact car is...

General: Fuel Economy The gas mileage (in miles per gallon) of a subcompact car is approximately

where x is the speed in miles per hour (for 35 ≤ x ≤ 65).

a. Find

b. Find and interpret your answers.

c. What does the sign of tell you about whether gas mileage increases or decreases with speed when driving at 40 mph? Do the same for and 60 mph. Then do the same for and 50 mph. From your answers, what do you think is the most economical speed for a subcompact car?