2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/72821A convex quadrilateral with sides a,b,c,d, and diagonals p,q is cyclic iff abp-bcq+cdp-daq=0. This condition, in spite of its simplicity, appears to be unnoted and unexpectedly proof-resilient. We employ advanced methods of computer algebra and nonlinear analysis.28 pp, 2 figures, A4 paper, in RussianGeneral MathematicsCommutative AlgebraMetric Geometry51K99, 13P99text