 Dear Marilyn, About Leap Years (Math Chat)  Frank Morgan, MAA Online
Your explanation [in Parade Magazine] of September 13, 1998 that we may need to skip a leap year EVERY FEW THOUSAND YEARS because the tropical year (about 365.2422 days) is a bit shorter than the average calendar year (365.2425 days) neglects CHANGES
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 Deberes matemáticas: Ejercicios y problemas de matemáticas resueltos paso a paso.  FORTEC, Formación y Tecnología S.L.
Herramientas didácticas específicas para la enseñanza y el aprendizaje de las matemáticas de la E.S.O. Ejercicios, problemas, exámenes, temarios, todos resueltos y explicados paso a paso. Por suscripción.
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 Decimal Places on a Number Line  NASA Lewis Learning Technologies Project
Decimal places: On a number line, which of the following numbers would be the farthest to the right? 0.001, 0.0015, 0.00099, 0.0005. A detailed answer is provided. From NASA's 9th Grade Math Proficiency Test.
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 Deep in the Jungles  Ivars Peterson (MathTrek)
Humans are predisposed to seeing order and meaning in a world that too often presents random choices and chaotic evidence. Random events sometimes get linked into a meaningful, though ultimately bogus, scenario... At the same time, the mathematics of
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 The Definitive Four Fours Answer Key  David A. Wheeler
From 0 up to 40,000, whole number mathematical expressions that use only common mathematical symbols and exactly four 4s. Wheeler offers his solutions in three different formats: an abbreviated list of answers; a text file of results, mathematical expressions,
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 Delphi For Fun  Gary Darby; Intellitech Systems Inc.
Delphi is based on the Pascal programming language developed in the early 1970s by Niklaus Wirth and named after mathematician Blaise Pascal. This site explores the use of programming as a tool in math problem solving, discussing interesting problems,
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 Delta Blocks  Hop David
A way to model different 3D tessellations and a tool for studying geometry, crystallography, and polyhedra. Delta blocks were inspired by M. C. Escher's print "Flatworms," which he said demonstrates that one can build a house not only with the usual
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 Department of Higher Mathematics, PhysicoTechnical Institute  Moscow State University
In Russian. Information on the department, study materials (textbooks and study aids by the dept. teachers, previous exams, lists of programs and classes and conferences for the current year), and information and download of a mathematical computer program
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 Department of Mathematical Sciences  U.S. Military Academy, West Point
Publications include Cryptologia, CASE (Computer Algebra Systems in Education), Mathematica Militaris, and PRIMUS (Problems, Resource, and Issues in Undergraduate Mathematics Studies). MathTECH is a research project that provides Java applets and TI89
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 Department of Mathematics and Computer Science  North Carolina School of Science and Mathematics (NCSSM)
Learning materials for teachers and students include resources on algebra, advanced functions, and mathematical modeling; data sets used with the NCSSM textbook Contemporary Precalculus Through Applications; the AP Calculus Web Center; and Mathematics
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 Derek's Daily Math  Derek Orr
"Every Number Has a Story to Tell." This blog by an undergraduate at the University of Pittsburgh details number theoretic properties for every day of the calendar year. Orr, a physics and math major, also maintains a list of "Interesting Propositions/Theorems
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 Desafios Matemáticos  Paulo Ferro
Cinco desafios novos cada mês. Ferro, um professor de matemática licenciado pela Universidade de Coimbra, tambem oferece demonstracões de geometria dinâmica, jogos matemáticos, biografias de matemáticos, exames
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 Des Trucs et des Maths  Philippe Picart
Sur le nombre pi; le nombre d'or; le Rubik's cube; des ressources à télécharger; et "des trucs de maths divers," comme un JavaScript qui répond à la question "Quel jour de la semaine estu né?" Avec un coins des
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 De Viribus Quantitatis  The University of Bologna
Luca Pacioli, a Franciscan monk, mathematics tutor, and colleague of Leonardo Da Vinci, wrote this foundational text of modern magic and numerical puzzles between 1496 and 1508. De Viribus Quantitatis (On the Powers of Numbers) contains the first ever
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 Dice and Valentine's Day Symmetries (Math Chat)  Frank Morgan; Christian Science Monitor
A dice challenge from S. H. Logue: The six faces of four dice have these numbers on them: A 1 1 5 5 5 5 B 4 4 4 4 4 4 C 3 3 3 3 7 7 D 2 2 2 6 6 6. You and your opponent each choose one die to use throughout the game. Each round you both roll, and the
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 Dictionary of Algorithms, Data Structures, and Problems  Paul E. Black
Originally compiled for the CRC Dictionary of Computer Science, Engineering and Technology: a dictionary of algorithms, algorithmic techniques, data structures, and archetypical problems, with related definitions. Algorithms include common functions,
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 Differential Equations (S.O.S. Mathematics)  Dept. of Mathematical Sciences, Univ. of Texas at El Paso
An online course: learning units presented in worksheet format review the most important results, techniques and formulas in college and precollege differential equations. Sections include: Introduction and First Definitions; Modeling via Differential
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 DigitalLesson.com  Mark Tully
Website for middle school math teachers that has middle school math lessons, games, projects and activities for use in the classroom. Free newsletter, other resources for sale.
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 Digital Math  Santa Barbara County Education Office
A site to assist Algebra I, Grade 7, and Grade 6 students, their parents, and educators with the California State Mathematics Content Standards. Log in for Quicktime that provides stepbystep instruction to solve problems aligned with the mathematics
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 Disaster Math  FEMA for Kids
Problems for kids about earthquakes, hurricanes, tornados, wild fires, winter storms, and floods. The site will get your score for you.
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 Discrete Math Problem of the Week (PoW)  Math Forum
Discrete math problems from a variety of sources, including textbooks, math contests, NCTM books, and puzzle books, and reallife situations, designed to reflect different levels of difficulty. From 1999 until 2002, this service challenged students with
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 Disfruta Las Matemáticas  Rod Pierce, mathsisfun.com
Spanish language version of Maths is Fun.
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 Dissections: Plane & Fancy  Greg Frederickson, Cambridge University Press
A book that explores an area of mathematical recreations, called geometric dissections, or the cutting of one or more figures into pieces that can be rearranged to form other figures. With an emphasis on using as few pieces as possible, these geometrical
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 The Distribution of the Knight  the Theoretical Research Institute
For every given starting point in the knight's tour problem, evaluate every
possible path that visits each square exactly once, and then count how many
solutions exist. Illustrations and calculations of the distribution of solutions
and dead ends across
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 Dividing 12foot Boards  NASA Lewis Learning Technologies Project
Lewis needs 1,500 boards to build a small rocket. Each board must be 3 feet long. If boards are sold in 12 foot lengths, how many boards must Lewis buy and cut into 3 foot pieces to get the 1,500 he needs? A detailed answer is provided. From NASA's 9th
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 Dividing Decimals  NASA Lewis Learning Technologies Project
Dividing decimals: What is 12.8 divided by .04? A detailed answer is provided. From NASA's 9th Grade Math Proficiency Test.
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 Dividing: Fractions and Mixed Numbers  NASA Lewis Learning Technologies Project
What is 1 1/2 divided by 1/2? A detailed answer is provided. From NASA's 9th Grade Math Proficiency Test.
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 Divisibility Rules  Calculation Tips & Tricks, The Math Forum
Tips for figuring out whether a number can be divided by the numbers 313, with a Spanish translation and an explanation of why the rules work by Robert L. Ward.
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 Divisible by 3  Andrew Stadel
Stadel has taught prealgebra, algebra, and geometry in middle school since 2004. His blog about "interactions with math, logic, life, teaching, and the web they spin together" dates back to March, 2012, with posts such as "Survey your students"; "2012
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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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 Division Worksheets  Guru Angad Educational Team
Overviews and freely downloadable PDF worksheets on math facts, math families, basic division, and long division, with stepbystep solutions.
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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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 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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 DO MATH...and you can do anything!  National Council of Teachers of Mathematics (NCTM)
Features grade levelspecific mathematical activities that children can do with their families and on their own, as well as links to other sites where children can explore the connections between math and some possible career paths. Also includes a family
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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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 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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 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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 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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 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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 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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 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. 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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 Dr. Vogel's Gallery of Calculus Pathologies  Thomas I. Vogel; Dept. of Mathematics, Texas A&M University
In learning calculus, students develop intuitive ideas of such concepts as limit, continuity, differentiability, and so on, useful in dealing with simple examples, but potentially a hindrance to deeper understanding of the basic concepts of mathematical
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 DSP First: A Multimedia Approach  McClellan, Schafter, & Yoder
The CDROM portion of the book, containing, for each chapter: Demos  quicktime movies, sound files, etc. that help reinforce the concepts introduced in the text; Labs  MATLAB based laboratory exercises for indepth study of certain concepts and ideas
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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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 Earning 5.5 Percent Interest  NASA Lewis Learning Technologies Project
Mareta put $4,000.00 in a savings account at an annual interest rate of 5.5%. How much interest will the principal earn in 1 year? A detailed answer is provided. From NASA's 9th Grade Math Proficiency Test.
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 Easier Fibonacci puzzles  Ron Knott
Puzzles that are simply related to the Fibonacci numbers....; Brick Wall patterns; Making a beeline with Fibonacci numbers; Chairs in a row; Stepping Stones; Fibonacci numbers for a change!; Telephone Trees ; Leonardo's Leaps; Fix or Flip; Two heads
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