IMO 1978 LL FIN13

The satellites A and B circle the Earth in the equatorial plane

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IMO 1978 LL FIN13

Origin: FIN

Problem

The satellites A and B circle the Earth in the equatorial plane at altitude h. They are separated by distance 2r, where r is the radius of the Earth. For which h can they be seen in mutually perpendicular directions from some point on the equator?