According to economists, this episode of great recession was resulted from a sudden busting of house bubble in US, where the house bubble was caused by rapid growth of improper regulation on sub-prime mortgages (Merriam 2009; Wang 2009; Whitney 2007).
When working with equations that have two variables, the coordinate plane is an important tool. It's a way to draw pictures of equations that makes them easier to understand. To create a coordinate plane, start with a sheet of graph or grid paper. Next, draw a horizontal line.
All four examples you gave have two variables -- u and v. In each of those examples u is a function of v. tiny-tim already answered for a situation in which there are three variables, where, for example, z is a function of two variables, x and y. So du/dx = 2xy2, and du/dy = 2yx2.