The reflective property of an ellipse is the principle behind "whispering galleries." These are rooms with elliptically shaped ceilings such that a person standing at one focus can hear even the slightest whisper spoken by another person standing at the other focus. Suppose that a dome has a semielliptical ceiling, 90 ft long and 24 ft high. Choose a coordinate system so that the center of the semiellipse is (0,0) with vertices (-49,0) and (49,0) and with the top of the ceiling at (0,24) .