Double points of two ranges on the same line.
Steiner's Double Element construction:
1) Double points on a circle : [url]http://www.geogebratube.org/material/show/id/35593[/url]
2) Double rays from a point: [url]http://www.geogebratube.org/material/show/id/35600[/url]
[b]→3) Double points on a line[/b]
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This is problem #60 in Heinrich Dorrie's [i]100 Great Problems of Elementary Mathematics[/i].
More: http://www.geogebratube.org/collection/show/id/2735
