 Doodling and Miracles  Cut the Knot!, Alexander Bogomolny
Almost 100 years ago, Frank Morley proved a curious theorem from elementary geometry that had remained unknown until 1899. With time, the theorem became known in mathematical folklore as Morley's Miracle (Morley's Trisector Theorem is a more mundane term.)
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 The DOR: Home of the Direct Opposite Reverse  David P. Sterner
Sterner developed this shape, which he calls a geometric "missing link", and has applications as a lens.
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 Doron Zeilberger's Electronic Headquarters
Doron Zeilberger researches combinatorics using computer programs. The site is notable for his Opinions page; these opinions are wideranging but touch upon mathematics education, the use of computers, and the value of incomplete proofs. All of Zeilberger's
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 Do software engineers need mathematics?  Keith Devlin (Devlin's Angle)
Software engineers don't use their college mathematics, but they make use of their college mathematics education every day. Brains are perhaps the world's best examples of an adaptive system. When we subject the human brain to an extended educational
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 Do the Math  Slate
A topical series about math in culture, politics, psychology, and other popular themes, many written by math professor Jordan Ellenberg. Articles, which date back to June, 2001, have included "Barry Bonds and the Placebo Effect," "Algebra for Adulterers,"
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 Dot Patterns and Sierpinski Gasket  Cut the Knot!, Alexander Bogomolny
How many ways are there to define the Sierpinski gasket (also, the Sierpinski triangle)? The author has counted a respectable 9 but would be happy to learn of more. An applet with rows of digits or circles with colors corresponding to the digits shows
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 Dots and Boxes  Ivars Peterson (MathTrek)
The familiar pencilandpaper game of DotsandBoxes can be played on several different levels of sophistication. Elwyn Berlekamp posits that the game can be played on at least four different levels. "Players at any level consistently beat players at
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 Double Bubble Conjecture Proved (Math Chat)  Frank Morgan, MAA Online
Four mathematicians have announced a mathematical proof of the Double Bubble Conjecture: that the familiar double soap bubble is the optimal shape for enclosing and separating two chambers of air. In an address to the Undergraduate Mathematics Conference
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 Doubling Pennies  Math Forum, a Classic Problem from the Ask Dr. Math FAQ
Which is more, a million dollars or a penny doubled every day?
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 Doubling the Cube  MacTutor Math History Archives
Linked essay covering the history of and some solutions to the classical
problem in Greek mathematics of doubling or duplicating the cube, also called the
Delian problem; and on proving the impossibility of constructing a solution using only ruler
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 Doug's Home Page  Douglas J. Ingalls
Includes mathematical interests such as images of fractal applications of Newton's Method and Gibb's Phenomena, with related programs in FORTRAN and BASIC, a discussion of the apparent depth problem in optics, and how multiple decisions can be reduced
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 Downloadable Articles, Papers & Materials  Michael de Villiers; Mathematics Education, Univ. of DurbanWestville, South Africa
Articles by Villiers related to geometry and geometry education some in PDF format, including: An Extended Classification of Quadrilaterals; A Mathematical Treasure Hunt; Place Kicking Locus in Rugby; Sketchpad Solutions and Results; A Dual, and Generalizations,
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 Downloads  Mac  Mathematics  CNET Download.com
Search this site to find a particular piece of software for the Mac, or browse their mathematics listings.
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 Downloads  PC  Mathematics  CNET Download.com
Search this site to find a particular piece of software for the PC, or browse their mathematics listings.
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 Do You Know That...  Interactive Mathematics Miscellany and Puzzles, Alexander Bogomolny
A collection of short essays about over 60 curious mathematical facts and terminology, mostly linked to explanatory or exploratory essays, drawn from various parts of Bogomolny's site, CuttheKnot. See page two and page three.
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 doyourmath.com
Interactive practice and learning tests (PreSAT, SAT, AP and Calculus), math puzzles, entertaining math for all ages, a math bookstore, and a math expert to answer your questions. Submit your answers to the puzzle of the week and see the archive of past
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 A Dozen Reasons Why Schools Should Avoid Filtering  From Now On (FNO)
Why schools should not block access to particular sites: filters may not work very well;. they may work too well; they may increase attention to SMUT and spur motivation; their ongoing cost may block better programs from your students; they may create
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 DPGraph (Dynamic Photorealistic Graphing)  David Parker
Software that allows you to: display multiple simultaneous dynamic photorealistic 3D implicit equalities and inequalities, and intersections of their volumes; spin, slice, and view timedependent (moving) photorealistic surfaces in real time; and construct
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 Dragonfly Web Pages  Miami Univ. of Ohio with the NSTA
Dragonfly is an inquirydriven magazine published bimonthly during the school year, inspiring children to join with their peers and with scientists in national and international investigations. Dragonfly also publishes poetry, autobiographies, natural
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 Draining The Swimming Pool (SMILE)  William Byrne, Martin Luther King High School
A lesson that offers experiments that suggest a way to solve the general work problem: If A can do a job in X hours and B can do the same job in Y hours, how long will it take A and B working together to complete the job? From the Algebra and Trigonometry
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 Dr. Alfred Posamentier  Education Update, Inc.
Blog by the Dean of Education at Mercy College (New York). Posamentier's Posts, which date back to August, 2010, have included "A Power Loop," "The Unusual Number Nine," "The Irrepressible Number 1," "Enriching Mathematics Instruction, Rather than Teaching
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 Dramatic Results
This nonprofit educational arts agency improves students' academic confidence and performance "by stimulating curiosity and fostering creativity through a handson, artsbased approach to learning." Dramatic Results specializes in curriculum that integrates
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 Drawing a Crop Circle  IES Ramon Cabanillas (Cambados, Pontevedra, Spain)
To draw a crop circle, the high school students at IES Ramon Cabanillas (Cambados, Pontevedra, Spain) calculated ellipses and discovered the pentagram's relation to the golden mean. This website details their designs, practice, calculations, and final
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 Drawing on Math  Tina Cardone
Blog by a public high school math teacher in Massachusetts who has participated in Boston University's PROMYS and the Institute of Advanced Studies' PCMI. Posts, which date back to August, 2011, have included "End of Year Reflections: Geometry Favorite
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 Drawing Tools  Shai Duvdevani
A growing group of .NET C# programs intended to make life easier for highschool and university students studying subjects such as math, computers and electronics. Each of these programs features saving and loading, so that sharing a drawing of logical
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 DreamCalc for Windows  Andy Thomas
A freeware scientific calculator for Windows, intended to replace standard handheld calculators. Fully featured range of math, scientific, statistical & financial functions. Supports natural fractions, complex numbers, basen logic, unit conversions
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 Dream Frequency .com
CGI web applications to calculate the factors of a number, calculate the prime numbers below a number, test a number for primality, calculate a word's position in the alphabetically sorted list, convert hex to ASCII, or convert temperature from celsius
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 Dream Home Mathematics (SMILE)  Dennis W. Montgomery, Lyman Trumbull Academy
A lesson for grades 512, designed to teach students to use basic math to determine affordability of a home based on a given income; calculate decimal placement for currency, percentages and interest; estimate affordability based on income; investigate
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 Dr. Genius  Hilaire Fernandes
Dr. Genius: "Geometry Exploration and Numeric Intuitive User System." The successor of the Dr. Geo Project, this GNU interactive geometry software allows the construction and manipulation of geometric figures. Read documentation, subscribe to the mailing
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 Dr. Hanley's Science Graphs  Patrick Hanley
A Windows software package for graphing data and equations in the form y=f(x) and x=f(th); y=g(th) using a PC. The program can also be used to graph up to 1,000 xy data points from an ASCII file, to graph color contour curves from equations and data
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 The Drills To Play  Cut the Knot!, Alexander Bogomolny
An exploration of a detail found at the beginning of Kurt Godel's biography by John Dawson, how repetition and practice constitute an integral and necessary part of the learning process; that, by implication, one first conceives an idea, then proves it
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 Dr Len Fisher: Opening the Door to Science  Len Fisher
Fisher concentrates on the science of the everyday and making science accessible. His latest book is Rock, Paper, Scissors: Game Theory in Real Life. The site contains excerpts from his books as well as descriptions of his talks, articles about his work,
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 Dr. Math Books  the Math Forum @ Drexel
Featuring Internet references, glossaries, indexes, illustrations, and questions written by actual kids, each can be purchased from Amazon.com via links on this page. The page details the contents of each book: Dr. Math Gets You Ready for Algebra, Dr.
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 Dr. Rich's Page o' Math
Math problems in algebra, trigonometry, and calculus, posted more or less monthly (past puzzles, with solutions, are on the Web).
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 Drums That Sound Alike  Ivars Peterson (MathLand)
Peterson writes that physicists and mathematicians have long recognized that the shape of the boundary enclosing a membrane plays a crucial role in determining the membrane's spectrum of normalmode vibrations. He outlines the progress made on Mark Kac's
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 Dueling Pinwheels  A Geometer's Sketchpad Activity  Cynthia Lanius
Construction directions for rotations, translation, and reflection, with questions to explore and a demonstration sketch to download.
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 Duke University Math Union  Jadrian Miles
The Duke University Math Union (DUMU) writes and hosts the annual Duke Math Meet. Download PDFs of problems and solutions from meets dating back to 1998.
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 dy/dan  Dan Meyer
Meyer has taught high school math since 2004, and blogged about it since September, 2006, with two interests in particular: curriculum design (answering the question, "how we design the ideal learning experience for students?") and teacher education (answering
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 Dynagraph  Rouben Rostamian, University of Maryland, Baltimore County
Dynagraph is a 3D plotting utility for the X Window System. It draws 3D curves and surfaces representing mathematical expressions and numerical data, and allows for interactive rotation, zooming, and other manipulations of the displayed images. Read the
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 The Dynamical Systems and Technology Project at Boston University  Robert L. Devaney; Dept. of Mathematics
An NSFsponsored project designed to help secondary school and college teachers of mathematics bring contemporary topics in mathematics (chaos, fractals, dynamics) into the classroom, and to show them how to use technology effectively in this process.
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 Dynamical Systems (Graphics Archive, Special Topics)  The Geometry Center
Modeling the timeevolution of the state of a physical system of quantities, attempting to understand all possible longterm behaviors.
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 Dynamic Geometry  Goldenberg, Scher; Education Development Center, Inc.
To understand how students construct mathematical ideas in "Dynamic Geometry" environments (tools like Geometer's Sketchpad and Cabri), one needs clear characterizations of the mathematical "raw materials" embodied in the software and how students use
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 Dynamic Labyrinths  Inge Schwank, Institute for Cognitive Mathematics, Department of Mathematics and Computer Science, University of Osnabrueck
Labyrinth construction sets to download or play online with Flash plugin.
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 Dynamic Number  KCP Technologies, Inc.
Classroomready activities of The Geometer's Sketchpad® for interacting with mathematical objects to develop deep understanding of number and early algebra concepts. Some of this content received support from the National Science Foundation (NSF)
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 Dynamic Web Tools Using webMathematica  Steve Wilson and Mike Martin, Johnson County Community College, Kansas
Interactive Web pages that allow students and faculty to compute and visualize mathematical results directly from a Web browser. Primarily for undergraduate courses, the pages are grouped by categories and a given page may be listed under several categories
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 Dynamische Geometrie Systeme (DGS)  Dr. Dörte Haftendorn
In German. Characteristics and comparisons of dynamic geometry systems; Euklid: characteristics, material, notes, software, lessons; TI92 calculator Cabri Geomètre materials. You may send for a 40page booklet on "Dynamic Geometry of Systems"
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 Dyscalculia and Maths Difficulties  Learning Works International Ltd
Conferences, teacher courses, in service educational training opportunities, research, and more from the publishers of Working with Dyscalculia.
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 Dyscalculia.org  R. M. Newman
Learn about dyscalculia: definitions, cognitive research, symptoms, diagnosis and best practices, classroom techniques, home learning aids, legal questions, papers such as "Developmental Perspectives on Dyscalculia: Implications for Teaching in the Middle
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 Dyslexia  George Row
A compensium of links to other WWW pages and discussion groups on Dyslexia: basics, organizations, bibliographies, archives, software, research; also special needs education in general; and pointers to general disability information.
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 Dyslexia Online
An online magazine offering information on and resources for dyslexia.
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