The document is a lesson on implicit differentiation and related concepts: 1) Implicit differentiation allows one to take the derivative of an implicitly defined relation between x and y, even if y is not explicitly defined as a function of x. 2) Examples are provided to demonstrate implicit differentiation, such as finding the slope of a tangent line to a curve. 3) The van der Waals equation is introduced to describe non-ideal gas properties, and implicit differentiation is used to find the isothermal compressibility of a van der Waals gas.