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Unformatted text preview: 5.1 Number Theory: Prime and Composite Numbers Number theory is primarily concerned with the properties of numbers used for counting. The counting numbers are also called the natural numbers: N = {1, 2, 3, }. The natural numbers that are multiplied together resulting in a particular product are factors of that product. If a and b are natural numbers, a is divisible by b , or b is a factor or divisor of a, if a b leaves a remainder of zero. This is symbolized by b a , b divides a. Rules of divisibility: 2 3 4 5 6 8 9 10 12 Last digit is 0, 2, 4, 6, or 8 Sum of the digits is divisible by 3 Last 2 digits are divisible by 4 Number ends in 0 or 5 Number is divisible by 2 and 3. Last 3 digits are divisible by 8. Sum of the digits is divisible by 9. Number ends in 0....
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 Fall '11
 PatriciaBishop

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