This lecture covers the reduction of higher-order differential equations to systems of first-order equations. The instructor explains how to define new functions and describe the resulting system. The focus is on the implicit function theorem, which provides conditions under which a curve can be locally represented as a function. Examples illustrate the application of the theorem to various equations, including circles and more complex functions. The lecture emphasizes the importance of derivatives and the conditions necessary for the existence of implicit functions. The instructor also discusses how to compute derivatives and the implications of these calculations in the context of differential equations. The lecture concludes with applications in thermodynamics, demonstrating the relevance of the implicit function theorem in practical scenarios. Overall, the session provides a comprehensive overview of the theoretical and practical aspects of differential equations and their solutions.