See also the
Dr. Math FAQ:
3D and higher
Browse Middle School Number Sense/About Numbers
Stars indicate particularly interesting answers or
good places to begin browsing.
Selected answers to common questions:
Multiplying by zero.
Casting out nines.
Number facts from the Web.
- Imaginary Numbers [02/28/1999]
What jobs and professions use imaginary numbers?
- The Importance of Number Sense and Estimating Answers [04/28/2004]
I don't understand how to solve problems using number sense. For
example, if an employee at the deli counter slices a 2 foot long
salami, about how many slices will he get? How can I figure that out?
Why is it important?
- Inequalities on a Number Line [05/07/2003]
How do you graph inequalities on a number line?
- Infinite Number of Primes [04/14/1998]
Prove there are an infinite number of primes. What is the largest prime
- Infinity [5/24/1996]
Is infinity positive or negative?
- Infinity and Inequalities [06/05/2002]
I'm supposed to use positive and negative infinity to solve problems
like 2x - 3 < 7.
- Infinity minus Infinity [10/26/1995]
Infinity - Infinity = ?
- Infinity Plus One [12/26/1995]
What is infinity plus one?
- Infinity times Infinity [10/15/1998]
What is infinity multiplied by infinity?
- Integers and Rational Numbers [5/25/1995]
Can you explain the difference between SET Q (rational numbers) and SET Z
- Integers in Everyday Life [01/16/2002]
How do we use integers in everyday life?
- Introduction to Rational Numbers for Middle School Students [12/06/2006]
Why do we need to know about rational numbers? Aren't all fractions
rational numbers? What's the difference?
- Irrational Between Two Rationals [07/19/2002]
How can I find an irrational number that lies between a pair of
- Irrational Numbers [08/13/1997]
How can three irrational numbers added together give you a rational
- Irrational Numbers [01/07/1998]
Are pi and the square root of 2 irrational numbers? Why? What about
1.33333... and 1.181818...?
- Irrational Numbers [01/13/1998]
How do you prove that (sqrt)2 is irrational? Given two irrational
numbers, a and b, is it possible to get an a^b that is rational?
- Irrational Numbers; Rational Square Roots [02/04/1998]
How can you tell whether root 10 is a terminating or repeating decimal,
or an irrational number? Are some square roots rational?
- Irrational Pi [7/15/1996]
Why is pi considered an irrational number when it's defined as the ratio
of the circumference of a circle to its diameter?
- Irrationals and Their Representations, Decimal and Otherwise [05/09/2015]
A teacher wonders if the non-terminating decimal expansions of SQRT2 and pi reveal
some inadequacy about the base ten number system — or her overreliance on
it. Expanding on her question, Doctor Peterson discusses the limitation of any human
- Is 0 a whole number? [7/17/1996]
Is 0 a whole number?
- Is Integer Division Associative? [05/14/2002]
Is integer division associative?
- Is It Possible That x/0 is Not Really Undefined? [06/14/2004]
Is it possible that 1/0 is not really undefined, but is in fact the
key to a whole new class of numbers, much as the imaginary unit i led
to the development of complex numbers?
- Isn't 1/0 the Biggest Number? [02/28/1998]
It makes sense that if you add or multiply a number to the biggest number
you will get the same biggest number, right?
- Is One a Triangular Number? [03/22/2004]
Triangular numbers have that name because if drawn as dots they can form a triangle. But 1 is just a single dot, so it can't be a triangular number, can it?
- Is Pi Infinite? [01/29/1998]
How can we know that pi is infinite? Does it ever turn into a pattern of
zeros and ones, like a computer code?
- Is Zero a Multiple of Every Number? [07/06/2002]
Is zero a multiple of every number? I have some books that say it is,
and others that say it isn't.
- Is Zero a Number? [10/23/1995]
Why is 0 classified as a whole number and not a natural number? Natural
numbers can be touched, whereas whole numbers cannot - is 0 a whole
number strictly by definition?
- Is Zero a Number? [07/05/2003]
If infinity is not a number, then is zero really a number?
- Is Zero a Signed Number? [01/09/2002]
What is the sign of zero?
- Is Zero a Square Number? [02/16/2000]
Is zero considered to be a square number? I have found some sources that
say it is, and others that say it isn't.
- Is Zero Even? [03/28/2001]
The justification that 0 is an even number is based on the fact that 0 is
divisible by 2. But how many times does 2 go into 0? Answer: no times; or
to rephrase, 2 doesn't go into 0. Can you explain this?
- Is Zero Even, Odd, or Neither? [03/31/1997]
Two students bet on whether zero is even, or neither even nor odd.
- Is Zero Odd or Even? [06/17/1998]
My 4th grade students are not satisfied with any explanation I can offer.
- Is Zero Prime or Composite? [10/23/1995]
Is zero prime, composite, or neither?
- Is Zero Really a Number or Just a Concept? [09/15/2005]
I've been taught that infinity is just a concept, not an actual
number. I'm wondering if the same thing can be said of zero?
- Karatsuba Multiplication [10/15/2007]
Can you explain the ideas and steps behind the Karatsuba method of
- K as an Abbreviation for Thousands [05/29/2002]
I was taught that 30K meant thirty thousand. Is that correct?
- Laps and Least Common Multiples [05/24/2002]
Three people are running around a track at different speeds. When
will they all return to the starting point at the same time?
- The Last Finite Number? [02/01/2004]
I just read an article about the "last finite number", and I was wondering whether such a number could possibly exist.
- Learning Addition and Multiplication with Algebra [3/16/1996]
How do you know what the (a) is in algebra? I saw this on a picture:
a(1/a) = 1/a(a) = 1...