Thirteen birds arrive and sit down in a plane. It's known that from each 5-tuple of birds, at least four birds sit on a circle. Determine the greatest Mā{1,2,...,13} such that from these 13 birds, at least M birds sit on a circle, but not necessarily M+1 birds sit on a circle. (prove that your M is optimal) geometrygeometric transformationreflectionalgebra solvedalgebra