Phenomena as diverse as the motion of the planets, the spread of a disease, and the oscillations of a suspension bridge are governed by differential equations. This course is an introduction to the mathematical theory of ordinary differential equations and follows a modern dynamical systems approach. In particular, equations are analyzed using qualitative, numerical, and if possible, symbolic techniques. MATH226 is essentially the edX equivalent of MA226; a one-semester course in ordinary differential equations taken by more than 500 students per year at Boston University. It is divided into three parts. MATH226.2x is the second part. For additional information on obtaining credit through the ACE Alternative Credit Project, please visit here.