Graphing Surfaces

A one-variable function draws a curve — a wiggly line above the x-axis. A two-variable function does something grander. Over every point (x, y) on the flat floor, it stacks up a height z = f(x, y). Connect all those heights and you get a SURFACE: a smooth landscape of hills, valleys, ridges, and saddles hovering in three dimensions. Graphing in 2D gave you curves; graphing in 3D gives you scenery.