The aircraft - Lycée Bellevue (Alès) Colegiul National Mihail Eminescu (Satu Mare - Roumanie)

An aircraft A flies in a straight line, at a constant height, with a constant speed v. On the ground an observer is watching on it. He is hearing no noise from the aircraft. But suddenly, after the aircraft passes above him, reaching the point B (see the image bellow), the humming noise of the aircraft reaches the observer's ears. If you know the angle 'alpha' between the direction OB and the horizontal, calculate the speed v of the aircraft.