 Divisibility Rules  Math Forum, Ask Dr. Math FAQ
How can you tell whether a number is divisible by another number (leaving no remainder) without actually doing the division? Why do 'divisibility rules' work?
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 Divisibility Rules to 50 and Beyond  Stephen D.T. Froggatt; Math Forum, Ask Dr. Math FAQ
A quick way of testing primality of larger numbers: the idea of deleting the last digit and adding or subtracting a multiple of the digit from the remaining number can be generalized to test for divisibility by prime numbers up to 50 and beyond.
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 Division by Zero  Dave Richeson
A blog about "math, puzzles, teaching, and academic technology." Posts, which date back to September of 2008 and sometimes introduce original applets, have included "Volumes of ndimensional balls," "Albrecht Dürer's ruler and compass constructions,"
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 Divisor Plot  Jeffrey Ventrella
"We hear of primes described as the 'building blocks' of all numbers. Let's turn that concept on [its] head. Instead, let's think of primes as the negative spaces behind complex objects. Imagine a series of picket fences stacked in front of each other.
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 DIY Calculator  Clive (Max) Maxfield and Alvin Brown
Order the book How Computers Do Math, or preview its table of contents. The book's interactive laboratories guide you in the creation of a simple calculator program to run on your DIY Calculator; its accompanying CDROM contains a virtual "DoitYourself
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 DLK MathsWork Software
Selfdifferentiating, topicspecific math software for Windows 3.1 and above, on number, shape and space, and data handling. All of the practice modules have on screen monitoring at three levels. Six problem modules designed to improve users' application
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 DNA Adds Up  Ivars Peterson (MathLand)
Calculating that 0 + 0 = 0, 1 + 0 = 1, and 0 + 1 = 1 is normally no big deal. When the calculations are done in the lab using DNA molecules, however, these elementary manipulations look considerably more interesting. In the July 12 [1996] Science, a team
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 DNA, Computers, and Killer Apps  Ivars Peterson (MathLand)
When computer scientists look at DNA, they see information storage and processing. They view the four bases of DNA as something akin to the zeros and ones of the binary code by which computers store and process information. They look for algorithms, theorems,
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 Dodecahedron Measures  Paul Kunkel
An investigation of the formulas of the surface area and volume for the dodecahedron. With icosahedron extensions.
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 Does March Madness need a timeout?  J. Travis  Science News Online
Suggestions about the effects of circadian rhythms, daily cycles of physiological activity that every organism experiences, and changes in time zone (causing 'jet lag'), on sports teams.
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 Does the Number e have Special Meaning?  Spencer, Chan; Univ. of Toronto Mathematics Network
Answers, explanations, and expositions of the question, Does the number e have any real physical meaning, or is it just a mathematical convenience? Addresses the topics: Simple and Compound Interest; A Physical Meaning for the Number e; The General Situation;
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 Doing Mathematics  Bryan Meyer
Blog by an eleventh grade math teacher at a projectbased learning (PBL) school pursuing a Masters Degree in Teacher Leadership. Posts, which date back to February, 2012, have included "What Bowflex Can Teach Us about Assessment," "Habits of a Mathematician,"
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 Domain and Range of a Function  Math Forum, Ask Dr. Math Common Question
A selection of answers to questions about the domain and range of a function, as well as determining the domain and range of a function from its graph.
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 Domebook  Tom Camilli
Cardboard and transparent plastic geodesic domes built by children. Students start by constructing paper models, then apply scale factors to construct domes of any size using cardboard or wooden dowels and plastic sheeting. See pictures of classroom projects;
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 Domino Artwork  Robert Bosch
Mathematical artwork of Bosch, who uses optimization techniques to create domino mosaics, flexible Truchettile mosaics, mapcolored mosaics, Traveling Salesman Problem (TSP) art, simpleclosedcurve sculptures, loops, labyrinths, and more. A professor
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 Dominoes Activity  Suzanne Alejandre
A classroom activity (similar to Pentomino + 1 = Hexomino; a variation on polyominoes) aligned to the NCTM and California Standards, to be explored through manipulatives (paper dominoes). Students explore whether it is possible to cover a 6x5 grid with
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 Donald Coxeter, Mathematician and Geometer  Great Canadian Scientists (GCS)
Harold Scott MacDonald Coxeter, Professor Emeritus, Math Dept., Univ. of Toronto, is best known for his work in hyperdimensional geometries and regular polytopes  geometric shapes that extend into the 4th dimension and beyond. In 1926 he discovered a
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 donalduncan's Channel
Narrated paper and pencil instructional videos  some, specific to British Columbia's Foundations of Math and Precalculus exams  on number, line segments and lines, rational expressions, factoring, quadratic functions, functions, exponents, polynomials,
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 Don Knuth's Home Page  Donald E. Knuth, Stanford University
From the author of The Art of Computer Programming. "Computer Musings" chronologically documents Knuth's informal lectures at Stanford. It also links to Musings Online, digitized versions of videos from the lecture series that included Dancing Links;
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 Don't Hate No Math  Elio Motella
Advice for students, parents, and teachers. The document called "Lecture" includes sections such as "The causes of your troubles," "It's called metacognition," and "Mental models and propositional representations"; see in particular Part D, titled "An
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 Doodal  Neil Thapen
Draw fractals freehand using a computer mouse: select a "brush size" to specify width in pixels, then click and drag in the large orange frame, the contents of which get copied in however many blue frames you specify. To rotate a blue box, click it while
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 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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