Solution
ID: past-exam-of-the-mathematics-course-of-the-university-of-cambridge/2008/ib/paper-3/12g/ii/solution
Past exam of the mathematics course of the University of Cambridge 2008 ib Paper 3 12G ii Solution by
Codex 0 Created 2026-09-24 Updated 2026-10-14
For the Poincare disc model, radial distance from zero to Euclidean radius isIt is minimal: any curve has length at least the integral of , which in turn is at least the net radial integral. Consequently a hyperbolic circle of radius centred at zero is , a Euclidean circle.
Given a centre , use a Poincare disc automorphism carrying to zero. Since it is an isometry, it takes the given hyperbolic circle to the preceding Euclidean circle. A Möbius transformation takes generalized circles to generalized circles. The preimage here is compact and contained in the disc, so it cannot be a straight line. ThusThis argument does not say that their Euclidean and hyperbolic centres coincide.
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