2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/64229We solve the following geometric problem, which arises in several three-dimensional applications in computational geometry: For which arrangements of two lines and two spheres in R^3 are there infinitely many lines simultaneously transversal to the two lines and tangent to the two spheres? We also treat a generalization of this problem to projective quadrics: Replacing the spheres in R^3 by quadrics in projective space P^3, and fixing the lines and one general quadric, we give the following complete geometric description of the set of (second) quadrics for which the 2 lines and 2 quadrics have infinitely many transversals and tangents: In the nine-dimensional projective space P^9 of quadrics, this is a curve of degree 24 consisting of 12 plane conics, a remarkably reducible variety.26 pages, 9 .eps figures, web page with more pictures and and archive of computations: http://www.math.umass.edu/~sottile/pages/2l2s/Algebraic GeometryComputational GeometryCommutative Algebra13P10, 14N10, 14Q15, 51N20, 68U05text