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 TOPICS This page:   number sense,   about numbers    Search   Dr. Math See also the Dr. Math FAQ:   number glossary and   integers,   rational numbers,   irrational numbers and   large numbers   and infinity and   prime numbers and   square/cube roots Internet Library:   number sense ELEMENTARY Arithmetic    addition    subtraction    multiplication    division    fractions/decimals Definitions Geometry    2-dimensional      circles      triangles/polygons    3D and higher      polyhedra Golden Ratio/ Fibonacci Sequence History/Biography Measurement    calendars/      dates/time    temperature    terms/units Number Sense/ About Numbers    infinity    large numbers    place value    prime numbers    square roots Projects Puzzles Word Problems Browse Elementary Number Sense/About Numbers Stars indicate particularly interesting answers or good places to begin browsing. Selected answers to common questions:     About zero.     Multiplying by zero.     Casting out nines.     Classifying numbers.     Googol, googolplex.     Number facts from the Web. Pentagon Puzzle [09/18/2002] Write the numbers 1 through 10 so each side of the pentagon has a sum of 14. Percentage of Votes Cast [06/03/2003] If you have two percentages, say 25% and 50%, can you do a percent change between the two? Perfect Numbers [9/15/1996] What is a "perfect number"? Pi's Last Digit [7/20/1996] I know pi is a nonterminating decimal, but if there were a last digit wouldn't the last digit be a 0? Placement of Commas in Writing Numbers [10/22/2003] Why is the ones period named for the ones place value and why is the thousands period named for the thousands place? Why is there not a hundreds period? Precise Numbers [10/18/2001] A precise number occurs when its proper divisors multiplied together equal the number: 6 is a precise number because 1*2*3 = 6. Is there another name or formula? Prefixes in Math [08/21/2001] What are the prefixes for 7, 8, 9, and 10? Prime and Composite Numbers, Sieve of Eratosthenes [01/28/1997] I need a list of all prime numbers and all composite numbers up to 50. Primes and Repeating Unit Numbers [12/09/1998] How do you prove this statement: For every prime number there exists a repeated unit number that is a multiple of that prime. Problem Solving and Mental Process [10/03/2002] Two numbers add up to 19. The difference between the two numbers is 3. What are the numbers? Proof: 2 = 1? [10/03/1997] a = b is factored to a^2 = ab by multiplying both sides by a... Proof that 1 + 1 = 1? [09/04/1997] Can you shed any light on this subject? Properties, Defined and Illustrated [10/10/2001] I need a page of information on the commutative, associative, and F.O.I.L. properties. Rational Numbers and Square Roots [10/07/1998] Our teacher told us to find all the rational numbers we could containing a sqrt symbol... The Real Number System in a Venn Diagram [09/08/1998] How do you construct a Venn diagram that illustrates the real number system? Reverse Polish Notation in Calculators [09/17/2005] An introduction to "reverse Polish notation" and the terms "stack", "push", and "pop" in that context. Roman Numerals [11/13/1997] What are three ways Roman numerals are used today? Roman Numerals on Clocks [06/04/2001] Is the Roman numeral on a clock written as IV, or IIII? Rounding to the Nearest Multiple of 5 [01/08/2005] How do you round to the nearest 5? Would 27 round down to 25 while 28 would round up to 30? Rounding to the Nearest Tenth [8/29/1998] Would 6.96 be 7.0 or 6.9? Saying Numbers Out Loud [03/06/2002] I have recently been told that the use of "and" is incorrect. Is it "one hundred one" or "one hundred and one"? Scientific Notation [09/16/1997] I don't understand how you get the answer: 5x10 to the 3rd power = 5,000; 5x10 to the -3rd power = -5,000. Scientific Notation of Zero [09/09/2002] What is the scientic notation of zero? Short Word Form [10/15/2002] How is 474,136 written in short word form vs. standard form? Significant Digits [09/07/1998] What are significant digits? Significant Digits and Irrational Numbers [09/24/2005] How can you determine the signifigant digits of a non-terminating or irrational number like pi? Significant Digits and Zero [05/19/1999] When are zeros significant and when are they not significant? The Size of Infinity [09/03/1998] What is infinity times 2? Can we measure infinity? Sizes of Infinities [01/31/1997] How can you prove that one infinity is larger than another? Smallest 5-Digit Number [02/05/2002] My dad says the smallest 5-digit number is 00000, but my teacher says it is 10000. The dictionary says that zero is a digit. What's the right answer? Spaces and the Number Line [02/26/2003] If you say, 'How many degrees are there between -2 and 4?' would you count the 0 degrees and answer 7 degrees or would you not count the nought and answer 6 degrees? Square Numbers End in 0 1 4 5 6 9 [10/15/2002] Why do all square numbers end in 0 1 4 5 6 9? Teaching Children Other Bases [02/20/2002] Why teach children number systems other than the base 10 number system? Teaching Elementary Concepts [04/04/1997] How can I teach rational counting, prime and composite numbers, and square roots at a grade-school level? Three Consecutive Natural Numbers [09/17/2001] Why is it that the product of any three consecutive natural numbers is always divisible by six? Trick for Numbers Divisible by 3 or 9 [02/24/1998] Proof of a trick for numbers that are divisible by 3 or 9. Trilogy, Tetralogy... [05/28/2003] Why isn't there a numbering system for groups like trilogies? Use and Meaning of 'And' in Naming Decimals [12/01/2004] Can you please tell me what the word "and" means in the word form of a decimal number? Use of Plural with Decimal Numbers [02/27/2002] What are the rules for 1.0001, 1.1, 1.5 or 1.999 ? Should we write 1.0001, 1.1, 1.5, 1.999 point or 1.0001, 1.1, 1.5, 1.999 points ? Differences between plurals in English and French. Using Integers [01/05/1999] I need some real-life examples of integers to show my students that they need to be able to use them in math class. Page: []

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