What is the cross-ratio of four lines $L_1,L_2,L_3,L_4$ if they are parallel? What is the cross-ratio if $L_4$ is the line at infinity?
What is the cross-ratio of four lines $L_1, L_2, L_3, L_4$ if they are parallel?
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geometry
geometric-construction
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1I know what the cross ratio of points. What is a cross ratio of lines? – 2012-06-27