This encyclopedia article describes how the credit card system works. It discusses the history, how credit cards are used, features, types, the numbering system, pros and cons, and more.

This page from Wikipedia, a free and open-source encyclopedia, covers how to solve a variety of trigonometric equations, most of which involve substitution with a trigonometric identity. The identities themselves are not covered, but...

This encyclopedia entry from Wikipedia offers a discussion on hyperbolic geometry. The site explains how the hyperbolic geometry plane is represented in two dimensions and how lines in the plane must be drawn. Other types of models for...

This encyclopedia article from Wikipedia on currency discusses the history and evolution of currency. It also discusses the code used to define currency, provides a list of currency names by country, and gives a listing of currency symbols.

Wikipedia, a publicly-maintained encyclopedia, provides an explanation of spherical geometry along with a listing of its most important uses.

Encyclopedia article about the Greek astronomer Hipparchus that contains information about his life and work. Also includes a lengthy list of references and links to additional information near the bottom of the page.

An encyclopedia article on differential equations discusses ordinary differential equations, how differential equations are solved, and uses of differential equations. The site has many links to clarify topics.

An encyclopedia entry on cluster sampling details what it is, how it is different from stratified sampling, and provides a discussion on geographical cluster sampling.

This site from the free encyclopedia, Wikipedia, provides a basic definition for plane of symmetry. Links are given for additional information on related subjects.

This site from the Wikipedia encyclopedia has formulas and examples of hyperbolas. A detailed graph and several formulas and examples are given along with several links to additional information on related subjects.

This encyclopedia article contains a basic description of what elliptic geometry is.