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Jeff Miller: Earliest Uses of Various Mathematical Symbols
This site from Gulf High School explores the earliest uses of various mathematical symbols, words, and terms. A very interesting site.
Missouri State University
Missouri State University: Introductory Statistics: Summation Notation
Provides a complete look at the mathematical summation sign. Examples of how to interpret and use the sign are presented as needed.
Interactive Mathematics
Interactive Mathematics: Roots and Radicals
Here you'll find information on radicals: notation, symbols, and laws for simplifying. The information is brief, but easy to follow.
Other
Dan's Math: Basic Skills Topics
College professor Dan Bach offers a review of various basic topics in arithmetic, pre-algebra, and algebra. Order of operations, exponents, square roots, variables, and combining terms are just a few of the many topics covered.
Wolfram Research
Wolfram Mathworld: Vinculum
A brief description of the viniculum, a part of the radical sign, notation for repeating decimals, distance between two points, and more.