"At what minimal height must be situated the point of reflection of a radio wave emitted in Montreal and received in Paris? The distance which separates both cities is 5 400 km and the radius of the Earth is 6 400 km." I don't even know where to start, we didn't learn this. I would assume the point is figuring the answer out by ourselves but I've been staring at the problem and all I could do was draw the situation. Please help! EDIT: Does it go something like this? http://imgur.com/5d7Mm EDIT2: I don't think it's an antenna, more like a satellite... I found... 6 400 km and that leaves me as confused as ever. Is it correct?