Paper 1, Section I, F

Geometry
Part IB, 2013

Let l1l_{1} and l2l_{2} be ultraparallel geodesics in the hyperbolic plane. Prove that the lil_{i} have a unique common perpendicular.

Suppose now l1,l2,l3l_{1}, l_{2}, l_{3} are pairwise ultraparallel geodesics in the hyperbolic plane. Can the three common perpendiculars be pairwise disjoint? Must they be pairwise disjoint? Briefly justify your answers.