notebook/notes/geometry/cartesian.md

---
title: Cartesian Coordinate System
TARGET DECK: Obsidian::STEM
FILE TAGS: geometry::coordinates
tags:
  - geometry
---

## Overview

In plane analytic geometry, the **Cartesian coordinate system** uniquely specifies a point by a pair of real numbers called its **coordinates**. These coordinates represent signed distances to the point from two fixed perpendicular oriented lines called the **axes**. The point where the axes meet is called the **origin** and have coordinates $\langle 0, 0 \rangle$.

%%ANKI
Cloze
The {$x$-coordinate} of a point is sometimes called its {abscissa}.
Reference: Tom M. Apostol, _Calculus, Vol. 1: One-Variable Calculus, with an Introduction to Linear Algebra_, 2nd ed. (New York: Wiley, 1980).
<!--ID: 1731184865785-->
END%%

%%ANKI
Cloze
The {$y$-coordinate} of a point is sometimes called its {ordinate}.
Reference: Tom M. Apostol, _Calculus, Vol. 1: One-Variable Calculus, with an Introduction to Linear Algebra_, 2nd ed. (New York: Wiley, 1980).
<!--ID: 1731184865791-->
END%%

%%ANKI
Cloze
The {origin} of a Cartesian coordinate system has coordinates $\langle 0, 0 \rangle$.
Reference: “Cartesian Coordinate System,” in _Wikipedia_, October 21, 2024, [https://en.wikipedia.org/w/index.php?title=Cartesian_coordinate_system](https://en.wikipedia.org/w/index.php?title=Cartesian_coordinate_system&oldid=1252434514).
<!--ID: 1731184876657-->
END%%

%%ANKI
Basic
Consider point $\langle x, y \rangle$. When does this point lie in the first quadrant?
Back: When $x > 0$ and $y > 0$.
Reference: Tom M. Apostol, _Calculus, Vol. 1: One-Variable Calculus, with an Introduction to Linear Algebra_, 2nd ed. (New York: Wiley, 1980).
<!--ID: 1731184865794-->
END%%

%%ANKI
Basic
Consider point $\langle x, y \rangle$. When does this point lie in the second quadrant?
Back: When $x < 0$ and $y > 0$.
Reference: Tom M. Apostol, _Calculus, Vol. 1: One-Variable Calculus, with an Introduction to Linear Algebra_, 2nd ed. (New York: Wiley, 1980).
<!--ID: 1731184865796-->
END%%

%%ANKI
Basic
Consider point $\langle x, y \rangle$. When does this point lie in the fourth quadrant?
Back: When $x > 0$ and $y < 0$.
Reference: Tom M. Apostol, _Calculus, Vol. 1: One-Variable Calculus, with an Introduction to Linear Algebra_, 2nd ed. (New York: Wiley, 1980).
<!--ID: 1731184865799-->
END%%

%%ANKI
Basic
Consider point $\langle x, y \rangle$. When does this point lie in the third quadrant?
Back: When $x < 0$ and $y < 0$.
Reference: Tom M. Apostol, _Calculus, Vol. 1: One-Variable Calculus, with an Introduction to Linear Algebra_, 2nd ed. (New York: Wiley, 1980).
<!--ID: 1731184865801-->
END%%

%%ANKI
Basic
The "vertical line test" of a Cartesian coordinate system is used to determine what?
Back: Whether the tested graph depicts a function or not.
Reference: Tom M. Apostol, _Calculus, Vol. 1: One-Variable Calculus, with an Introduction to Linear Algebra_, 2nd ed. (New York: Wiley, 1980).
<!--ID: 1731185808236-->
END%%

%%ANKI
Basic
In Cartesian coordinate systems, why does the vertical line test work?
Back: A function is single-valued. A vertical line that intersects a graph multiple times immediately contradicts this.
Reference: Tom M. Apostol, _Calculus, Vol. 1: One-Variable Calculus, with an Introduction to Linear Algebra_, 2nd ed. (New York: Wiley, 1980).
<!--ID: 1731185808243-->
END%%

## Cartesian Equations

An equation that completely characters a figure within the Cartesian coordinate system is called a **Cartesian equation**.

%%ANKI
Basic
What is a Cartesian equation?
Back: An equation that completely characterizes a figure within the Cartesian coordinate system.
Reference: Tom M. Apostol, _Calculus, Vol. 1: One-Variable Calculus, with an Introduction to Linear Algebra_, 2nd ed. (New York: Wiley, 1980).
<!--ID: 1731185808246-->
END%%

%%ANKI
Basic
What is the Cartesian equation of a circle centered around the origin with radius $r$?
Back: $x^2 + y^2 = r^2$
Reference: Tom M. Apostol, _Calculus, Vol. 1: One-Variable Calculus, with an Introduction to Linear Algebra_, 2nd ed. (New York: Wiley, 1980).
<!--ID: 1731185808249-->
END%%

%%ANKI
Basic
What figure does the following Cartesian equation characterize? $x^2 + y^2 = r^2$
Back: A circle with radius $r$ centered around the origin.
Reference: Tom M. Apostol, _Calculus, Vol. 1: One-Variable Calculus, with an Introduction to Linear Algebra_, 2nd ed. (New York: Wiley, 1980).
<!--ID: 1731185808252-->
END%%

## Bibliography

* “Cartesian Coordinate System,” in _Wikipedia_, October 21, 2024, [https://en.wikipedia.org/w/index.php?title=Cartesian_coordinate_system](https://en.wikipedia.org/w/index.php?title=Cartesian_coordinate_system&oldid=1252434514).
* Tom M. Apostol, _Calculus, Vol. 1: One-Variable Calculus, with an Introduction to Linear Algebra_, 2nd ed. (New York: Wiley, 1980).