René Descartes (1596-1650) was a French philosopher and mathematician. As a philosopher, he is famous for the saying “Cogito ergo sum” (“I think, therefore I am”), and his writings led many to consider him the Father of Modern Philosophy. Even today, a number of his writings are standard faire in university philosophy departments.
However, it is Descartes’ work in mathematics that form the basis for this chapter, particularly his invention of the Cartesian Coordinate System which bears his name. Descartes’ invention of the coordinate system created an entirely new branch of mathematics called analytic geometry, which established a permanent link between the plane and solid geometry of the ancient Greeks and the algebra and analysis of modern mathematics. As a result of his work, mathematicians were able to describe curves with equations, unheard of before Descartes’ invention of the coordinate system. Rather than describing a circle as the “locus of all points equidistant from a given point,” mathematicians were now able to refer to a circle centered at the point (0, 0) with radius \(r\) as the graph of the equation \(x^2 + y^2 = r^2\).
The bridge created between geometry and analysis as a result of Descartes’ methods laid the groundwork for the discovery of the calculus by Newton and Leibniz. For his efforts, mathematicians often refer to Descartes as the Father of Analytic Geometry.
In this chapter we will introduce readers to the Cartesian coordinate system and explain the correspondence between points in the plane and ordered pairs of numbers. Once an understanding of the coordinate system is sufficiently developed, we will develop the concept of the graph of an equation. In particular, we will address the graphs of a class of equations called linear equations.