After leaving the airport, the aircraft takes off at an angle of [[$ 9 ° $]] relative to the ground. The aircraft will start a horizontal flight when it has reached an altitude of [[$ 4000 \;\text{m} $]]. How many kilometers from the airport has it flown when this happens, measured along the surface of the Earth? (Source: Entrance exam for engineering education, spring 1992)

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Answer: The plane km from the airport.