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. MATH226x is an introduction to the mathematical theory of ordinary differential equations. This course follows a modern dynamical systems approach to the subject. 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. MATH226.3x is the last part. For additional information on obtaining credit through the ACE Alternative Credit Project, please visit here.